Torvalds Has Harsh Words For FreeBSD Devs 571
An anonymous reader writes "In a relatively technical discussion about the merits of Copy On Write (COW) versus a very new Linux kernel system call named vmsplice(), Linux creator Linus Torvalds had some harsh words for Mach and FreeBSD developers that utilize COW: 'I claim that Mach people (and apparently FreeBSD) are incompetent idiots. Playing games with VM is bad. memory copies are _also_ bad, but quite frankly, memory copies often have _less_ downside than VM games, and bigger caches will only continue to drive that point home.' The discussion goes on to explain how the new vmsplice() avoids this extra overhead."
Wrong Side of Bed? (Score:5, Insightful)
Do I have that right?
If so, I'm not really seeing his issue. Or at least not as hard-line as he sees it. The issue of memory copy performance is a tricky one, especially since CPU cycles are not the be-all to end-all of performance. Does the exception generated really cost that much more than he believes, or is it often eclipsed by the cost of the extra memory read/writes and CPU waits that are normally generated by a copy? Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?
I'm just not sure that Torvalds is really looking at all sides of this. He may be right, but I'd like to hear more discussion between the *BSD guys and Torvalds before we put this matter to rest. And preferrably without the insults this time.
Links:
Copy on Write as explained by Wikipedia [wikipedia.org]
FreeBSD page on Zero Copy Patches [freebsd.org]
Duke Uni Research [duke.edu]
Wrong side of compiler (Score:5, Funny)
Re:Wrong side of compiler (Score:5, Interesting)
The only microkernel Linus knows jack about is Mach, an ancient piece of crap, which indeed is Linus indeed calls it. It's unfortunate real-world systems were saddled with it, and it's got real performance issues, but Linus carries on about it like Mach ran over his dog or something.
He conveniently ignores or chooses to remain ignorant of the fact that L4Linux is typically faster than Linux itself. To say nothing of the real-world success of QNX. And even L4Linux is pretty old by today's standards.
This is all pretty typical behavior of Linus: bluster now, bone up and learn, and implement it later. He did so with SMP (saying famously that the way to do it was one Big F**ing Lock, then learning that no this wasn't such a great idea after all). Then he went on a tirade about sun's
Ultimately, Linus and Linux come around. Sometimes he just has to vent.
Re:Wrong side of compiler (Score:3, Informative)
Re:Wrong side of compiler (Score:4, Informative)
You're quite mistaken. L4Linux runs Linux in usermode on top of the L4 kernel.
http://os.inf.tu-dresden.de/L4/LinuxOnL4/ [tu-dresden.de]
What harsh words? (Score:5, Insightful)
Linus said that he felt this was wrong, and that being a prof is no excuse for Minix being the mess it was (and Minix was a mess in the late 1980s/early 1990s). He also apologized if he came off as too harsh for his writing about how people should be able to throw away an old design in favour of a new one anyway, etc.
It was very polite compared to some of the non-Andy/Linux replies.
Re:Wrong side of compiler (Score:3, Informative)
Re:Wrong side of compiler (Score:5, Insightful)
More likely he had some really bad acid the previous night.
After all he did more than 6 revisions of the Linux VM using CopyOnWrite before this latest fad.
Possibly more.
Off the top of my head that is at least 1 in the 1.2 tree, 1 in the 2.0 tree, 1 in the 2.2 tree, 2 in the 2.4 tree and more than 2 in the 2.6 tree, all of which being CopyOnWrite and at least some of which has been hailed as the next best thing after hot bread.
As far as the technical point he is possibly correct for x86 where COW goes through the fault mechanism and causes some TLB and cache abuse which is really bad on modern CPUs. I am not sure as far as other architectures are concerned, because IIRC (I may be wrong) the memory mapper hardware on the old Sparc was designed for COW in first place.
Anyway, before calling somebody else an idiot for something you have happily done for 10+ years till yesterday it may be nice if you look at yourself in the mirror. Because I never remember any branch of FreeBSD reaching the point where you can do a find /usr -exec cat {} > /dev/null \; to hang the system. That is 2.6.16 at your service (from rc4 onward) on at least two x86 subarchitectures where I had the time to test it. That is besides the unkillable processes in [S] state on an nfs flock in 2.6.14 (yep, that is a gem which no other unix has managed so far), besides the OOM idiocies in 2.6.10, besides deliberately making it absolutely impossible to backtrack any more interesting patch to a previous kernel without employing a team of kernel developers because the VM and locking is not compatible across any kernel version since 2.6.9 and even when it is something else is changed like the tty layer, besides.... Aarghh.....
Re:Wrong side of compiler (Score:5, Funny)
Re:Wrong side of compiler (Score:5, Funny)
i know what's the last book [alphabetofmanliness.com] that linus read. do you?
Re:Wrong Side of Bed? (Score:5, Insightful)
Re:Wrong Side of Bed? (Score:5, Informative)
Well, I am an expert in kernel programming, and I can tell you that Linus has little tolerance for anyone who doesn't program the way he does. That's one reason, for example, that he doesn't support debuggers. Every other OS has a kernel debugger built-in (and therefore, generally stable and full-featured), but not Linux. Even the OS/2 kernel debugger that was created 10 years ago is better than anything Linux has.
Re:Wrong Side of Bed? (Score:3, Insightful)
How can you say something like this? If Linux had a debugger from the start, it could be ripped out right now if there was some gain by doing that. By not having it, you only induced developers lots of pain during the last 10-15 years, for those occasions where a debugger really are the right tool for the job.
And yeah, I know some of Linus' theories about how to program, how he thinks asserts and invariants are bad things, I just don't agree with
Re:Wrong Side of Bed? (Score:2)
Re:Wrong Side of Bed? (Score:4, Funny)
Re:Wrong Side of Bed? (Score:5, Insightful)
No it won't. The only way to avoid the copies is to avoid the pagefaults. Since userland doesn't get explicit notification in FreeBSD of when the pages are safe to use, the process should wait as long as possible (e.g. until malloc() starts failing)
The idea Linus pushes here is explicit notification- via select() or poll() and returnable via recvmsg(). That way the userland knows exactly which pages can be reused.
The result is that it's faster and easier to develop userland programs to take advantage of it. It's also easier to degrade gracefully into read()/write() until the FreeBSD people see the light and add support for this too.
It's really a clever idea.
Re:Wrong Side of Bed? (Score:4, Insightful)
Right. Especially with multithreaded apps as Linus pointed out. Also the TLB misses could get expensive as well, and again the TLB misses will be more of an overhead with multithreaded apps.
I don't believe that COW is completely evil. It exists, obviously for a reason, but I would agree with Linus on a much less harsh tone (depending on mood).
Oh, and isn't "VM" a trick to begin with?
Re:Wrong Side of Bed? (Score:5, Informative)
No. Updating the page tables twice and having a fault in there is very expensive.
Linus believes that the exception will occur enough in real world usage that it will be slower than just doing the copy in the first place.
And he's right too. But he's not recommending the copy "in the first place" - he's recommending explicit notification that the pages aren't used anymore instead of an implicit notification by-way of a page fault.
Linus wants to push the manual use of zero-copy memory sharing through the vmsplice() routine. He believes that the programmer will always know better than the system when to share memory.
That's correct.
Does the exception generated really cost that much more
Yes. There isn't a grey area on it either- it's basic math: cost of page copy + exception + 2 * (page table update) is greater than cost of page copy + page table update.
The real issue is that the userland knows what it's doing. Eventually it'll want to reuse a buffer. Now does the userland start reusing pages when malloc() fails- thus incuring the exceptions when memory is tight? Or does it reuse them when the kernel says they're reusable?
The latter makes more sense if you're actually concerned about performance. The former may be easier to code, but I doubt many people will actually do that because it's hard to test.
In practice what people do is use a static buffer- that's even EASIER to code, but it means page faults happen ALL the time.
Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?
They already have to do it. Whether it's the BSD implementation or the new Linux implementation they already have to do it if they want reasonable performance in the real world.
To really take advantage of the BSD implementation, your program needs to monitor malloc() usage, and start attempting to reuse pages when it fails- oldest to newest. This is complicated and hard to test.
To really take advantage of the Linux implementation, your program waits until it gets notification (via select() or poll()) on the vmsplice() recvmsg() operation. Once that occurs, the notification says exactly which pages can be used.
The result? Userland on Linux is easier to write, and easier to test. It'll also be faster.
Re:Wrong Side of Bed? (Score:3, Interesting)
Re:Wrong Side of Bed? (Score:5, Insightful)
Funny that we just had an article about how many Linux users and enthusiasts exclude other people by being complete dicks, and here you are, acting like a dick. Of course, I don't know you from Joe Blow, so maybe I just misunderstood your obviously angsty response.
"That's obvious."
"Correction: when _you_ start using up a lot of memory Linux totally sucks. When I start using up a lot of memory, Linux acts exactly as I expect, and better than FreeBSD."
Linux acts exactly how I'd expect, too. It completely sucks when it comes to memory and process management. Linux may have a better threading kernel, but that's the only thing that seems to save it in the real world. After only six years of administering servers professionally and for my own use, it has come down to Linux on the desktop, and FreeBSD for Real Work(tm). Many large companies that depend on their data agree with me, and those who use Linux or Windows just throw more machines at the problem.
At least Linux is free compared to Windows, right?
Re:Wrong Side of Bed? (Score:5, Insightful)
...but still inferior to Windows, right? I mean, we're only looking at number of installations, after all. Furthermore, McDonald's clearly has the best hamburger and Velveeta beats Hoffman's Super Sharp.
I like Linux. I'm typing this on a Gentoo box. However, I'd never pretend that it's better in every single aspect than any other OS in existence. The BSD guys have a few tricks up their sleeve, and even Redmond manages to get things right on rare occasion.
Re:Wrong Side of Bed? (Score:2)
There you're assuming that the page copy will be necessary. In cases where the W in COW does NOT occur, isn't COW much better?
(Please don't mod me up. I have no idea what I'm talking about.)
Re:Wrong Side of Bed? (Score:3, Informative)
Well, no, it's about the same actually.
The problem is that in the naive implementation, the page copy is always necessary. A complicated implementation (in userspace) to take advantage of COW is more complicated than with explicit notification.
Re:Wrong Side of Bed? (Score:3, Interesting)
That's certainly not the impression I get from Dave Miller's commentary about splice/tee to sockets, which discusses using poll/select/more advanced methods to see when the splice has finished and comments:
Re:Wrong Side of Bed? (Score:4, Interesting)
However, given that the "free()" routine is part of the OS in FreeBSD, wouldn't it make sense to create a smarter "free()" routine that would attempt to recognize and explicitly deallocate CoW pages?
Re:Wrong Side of Bed? (Score:3, Interesting)
Oh no. That's the solution actually.
The problem is in using a static buffer instead of allocating a buffer for each send operation. If you use a static buffer, you ALWAYS cause a fault. If you malloc() each time, you won't fault- at least until you reuse the pages later (when malloc() fails).
However, given that the "free()" routine is part of the OS in FreeBSD
No, it's not unfortunately. It's
Re:Wrong Side of Bed? (Score:5, Informative)
* Lightbulb goes on
Oohhhh, I see! So something like this is the problem:What you're saying is that every time through the loop, there's going to be a page fault as the CoW pages are wiped away by the new copy into the same logical buffer. CoW is dependent on allocating new pages every time so that you don't ever write to the old CoW pages. Correct?
Of course, this is where I'd really like to hear from the *BSD developers. Surely they must be aware of this issue? Do they expect programmers to throw away their buffers, or do they have a plan?
Re:Wrong Side of Bed? (Score:5, Informative)
Exactly correct. Those frequent CoW operations are slow- the page faults are expensive. If you had instead written:
char *buffer;
int read = 0;
int length;
while(read < totalSize)
{
buffer = malloc(1024);
length = fread(buffer, 1, 1024, &file);
read += length;
}
Then it would operate quickly on FreeBSD. The problem then becomes exactly when do you free all those malloc()s?
On Linux, you can get a signal from the kernel- via a recvmsg() call that will tell you exactly which pages are now available to be freed- or better still, reused.
It'll be easy to check and test correctness AND the programmer has to be aware it's going on in order to use it at all.
Under FreeBSD the programmer can use the syscall, but never get the performance unless they know exactly what's going on.
Of course, this is where I'd really like to hear from the *BSD developers. Surely they must be aware of this issue?
I don't know. The article wasn't about that- I doubt Linus pays attention to what the BSD people know- in fact, I don't even think he knows for certain if FreeBSD even works this way.
The point is that using CoW is stupid for this. It makes things complicated in the hard case, and in the easy case, it makes things slower.
Re:Wrong Side of Bed? (Score:4, Informative)
Then it would operate quickly on FreeBSD. The problem then becomes exactly when do you free all those malloc()s?
No, it'd be slower than just copying on FreeBSD too.
while(read < totalSize){buffer = malloc(1024);
length = fread(buffer, 1, 1024, &file);
read += length;
}
This is where VM games really bite you in the ass, because you get false sharing. Even if you never reuse the buffer, this can cause 3 copies--each group of 4 (3.99ish) buffers will be on the same page, and therefore each call will cause a fault from the previous one.
In theory the OS could be allow itself write & check for overlapping calls (& avoid the COW fault), but note that the read() example really isn't interesting for zero-copy unless you're using hardware TCP offloading. Zero copy is more interesting for write(). The usual case is then:
while(){b = malloc();
fill_in_buffer(b);
write(b);
}
and that fill_in_buffer step *must* cause a fault if sets of buffers are on the same page. To avoid COW faults you have to be really careful that you don't accidentally write to the same page as the buffer--even indirectly by malloc updating it's inline data structures. That's pretty nasty to do--the easiest way is to allocate 8K at a time, and use a page-aligned chunk from the middle of it. Talk about a waste of memory.
Re:Wrong Side of Bed? (Score:5, Informative)
This isn't about fork() it's about zero copy buffers, not code and data pages in general.
Consider a block like this:Now, on the whole, if zero_write() works like write() then an awful lot of copying is going on. But if zero_write() uses the buffer for kernel space as well, it's much faster (1 less copy).
Now the trick is returning to userspace before the buffer is completely used. In FreeBSD a page fault would occur immediately during read().
Both FreeBSD and Linux agree that you shouldn't do this. Instead something like this:The trick at this point, is that elsewhere in your code, Linux can tell you when those malloc() buffers can be reused, whereas FreeBSD doesn't. It relies on the fact that you'll either make a blocking call on fd2 before you free buffer _OR_ you'll accept a page fault.
But if you can be told when it will occur, you don't need to do either of these things, and as a result, you NEVER have to wait. This means your program will be simpler and go faster.
Re:Wrong Side of Bed? (Score:4, Interesting)
>> through the vmsplice() routine. He believes that the programmer
>> will always know better than the system when to share memory.
>
> That's correct.
No, that is not always correct.
I am a C developer for a large multinational corporation that likes to make money. When I need to fork(), I do not have the time to think of all the memory management invovled with fork(). I just want it to be done reliably, and I want it to be done fast.
If it turns out that my code runs 10% faster on FreeBSD than on Linux, than that means that the code is probably going to go on a FreeBSD system. And if FreeBSD is not an option, than I am not going to do the optimization (because CPUs cost less than my wages).
Also: optimization never happens anyways (or at least, not properly).
So from my perspective:
I want the kernel to run my code as fast as possible by default.
Re:Wrong Side of Bed? (Score:3, Informative)
So what? Who's talking about fork()?
This is about copy-on-write of zero-copy fifos and TCP. If you don't know what the rest of us are talking about, please just say so, and we'll be happy to tell you exactly what's going on.
Maybe you'll have something to contribute at that point, or maybe you'll just learn something.
And if FreeBSD is not
Re:Wrong Side of Bed? (Score:5, Informative)
This has NOTHING to do with fork(). You are used to CoW (copy-on-write for anyone else reading along) only applying to fork(), but that is not the issue under discussion at all. You, and probably 95% of the responders here, need to go RTFA.
The issue is implementing zero-copy IO. FreeBSD's way of doing it do a setsockopt() that causes any write() on that socket to mark the buffer CoW so that it can use it exclusively for handing down to the device driver. The "magic" is that if the programmer tries to use that buffer while the device driver owns it he will get a copy. BUT, the programmer has no way of knowing when that buffer is available again.
Linus's point is that marking a page CoW is very expensive - especially in an SMP environment, almost as expensive as just copying that page to begin with would be. He also argues that taking a page-fault to invoke the CoW to a new page, or simply to turn off the CoW attribute, is orders of magnitude more expensive than just copying it in the first place.
So that means the CoW for sockets is only really useful if you rarely or never reuse your buffers again. And the only place that happens is in synthetic benchmarks.
If Linus had said "Microsoft is a bunch of idiots for implementing a feature that only looks good on benchmarks" everybody would be nodding their heads in agreement. I think the reason people are not doing the same here is because they just don't understand the details.
Re:good approach (Score:4, Informative)
No, it has a speed advantage over read()/write() provided you are aware of exactly how it works. The fact that it's transparent to the userspace is a bad thing because it means you have code written a certain way- that nobody will ever understand why.
Reusing the pages causes the speed benefit to go away- and in fact it'll be slower than read()/write().
This sort of thing matters almost exclusively to people doing really deep performance tuning, and for them it's better to present a simple API with large rewards for tuning, instead of transparently doing something weird to an existing API that will break in the field without you noticing and requires really weird usage to get the best performance.
I agree completely. Unfortunately, the FreeBSD API is inadequate. It's not faster in practice unless you do something really really weird (waste memory). The big difference is the Linux implementation gives explicit notification and the FreeBSD API doesn't.
FreeBSD doesn't provide an API to ask if the pages are still in use. That'd probably make their approach usable- but at that point, why bother updating the page tables at that point?
Once you're there, why bother statpage() to check to see if the page is in use? Why not have the kernel send the pages that are available via a file descriptor so you can poll() or select() on it?
At this point, you're at the Linux implementation.
That's it. That's why it's better.
Re:Wrong Side of Bed? (Score:4, Funny)
If there is something that FreeBSD does that he likes, he is welcome to the code. If there is something that FreeBSD does that he does not like, he can just let it go.
Why does he feel the need to start a war within the OpenSource community?
Re:Wrong Side of Bed? (Score:5, Funny)
Re:Wrong Side of Bed? (Score:5, Insightful)
He's not saying "The FreeBSD people should rewrite that part of their OS," he's saying "don't put that crap into the Linux kernel."
Re:Wrong Side of Bed? (Score:5, Interesting)
The issue of memory copy performance is a tricky one, especially since CPU cycles are not the be-all to end-all of performance. Does the exception generated really cost that much more than he believes, or is it often eclipsed by the cost of the extra memory read/writes and CPU waits that are normally generated by a copy? Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?
What programs weigh in at hundreds of megs? Don't count data files or map files for games. The entire bin directory of a PostgreSQL install is only 20 megs, and that's a lot of stuff there.
And as far as doing memory management... YES. I have yet to see a compiler do a better job at managing memory than what I can do when I write my code - and the reason is quite simple: I'm the domain expert, not the compiler. Compilers generally do a good job, but it's those specific cases that bite you over and over again.
Linus is also right about child threads writing to memory. If that never happened, we wouldn't have a concept of a lock or a semaphore. The bottom line is that is happens a lot.
He may be right, but I'd like to hear more discussion between the *BSD guys and Torvalds before we put this matter to rest. And preferrably without the insults this time. :-/
I agree, the ad hominem was completely unecessary.
Re:Wrong Side of Bed? (Score:2)
I think the problem with this approach is that COW will only give you a copy of the particular piece of the memory that you accessed. That means that
Re:Wrong Side of Bed? (Score:3, Interesting)
It doe
Re:Wrong Side of Bed? (Score:3, Interesting)
What Linus suggests is explicit notification- say a select() or poll() operation that says "these pages are now free". This works out well, and is indeed faster because there aren't any copies or page faults. It's also easier to develop.
Problem is that unless you're talking about declaring the pages "free" by storing more data in the heap info structure, declaring the pages free would require trapping into the kernel, and that is every bit as slow as the exception on most architectures, only now you're
Re:Wrong Side of Bed? (Score:3, Informative)
No. System calls are not as slow as exceptions.
If they are on your architecture, you're not supporting a million clients at a time on that architectur
Re:Wrong Side of Bed? (Score:3, Interesting)
Copy on Write saves you real memory, cache memory, and CPU time by pretending that each forked process has a true copy of a memory segment when it in fact is looking at the original. That is, right up until a fork tries to write to that memory location, in which case an exception is handled by making an actual copy to a new location and allowing the write.
Linus believes that the exception will occur enough in real world usage that it will be slower than just doing th
Re:Wrong Side of Bed? (Score:3, Informative)
OG.
Re:Wrong Side of Bed? (Score:3, Informative)
The real problem is that in a zero-copy world, write() returns before the data is sent, and in FreeBSD there is no way for the kernel to signal the user program that the write() is complete and it is safe to re-use the buffer
--jeff
RTFA, please. Or at least my summary here. (Score:5, Informative)
Basic explanation: Suppose that a program is doing a lot of output to a file or socket. The program can generate data faster
than the kernel can consume it, say. So what should the kernel do with the buffer it receives from the user on each write()?
There are three options.
1) Copy its content immediately elsewhere, so that on return to User Mode, the buffer remains writable and writes are safe.
2) Change the access rights of the page containing the buffer, so that no copy need be made unless User Mode attempts
to modify its content before the kernel has completed the write(). If the user attempts to write, it either gets
permission to do so (because the kernel is done) or it gets a writable copy.
3) Let User Mode promise to not modify the buffer's content until told that it's safe to do so, leaving it writable in
the meantime.
The default behavior is (1); BSD's zero copy socket feature is (2), and the point of Torvalds' complaint; vmsplice() is (3).
Re:RTFA, please. Or at least my summary here. (Score:4, Insightful)
Re:RTFA, please. Or at least my summary here. (Score:3, Insightful)
Re:RTFA, please. Or at least my summary here. (Score:3, Insightful)
Re:Wrong Side of Bed? (Score:3, Funny)
Debian GNU/kFreeBSD (Score:2, Offtopic)
Playing games with VM is bad. (Score:3, Funny)
I know, I hate it when I have to listen to 26 hang up messages in my inbox only to find out someone is playing games with me.
I call bullshit! (Score:4, Funny)
Re:I call bullshit! (Score:2)
That's what the calls were... (Score:2)
He didn't say they were legitimate messages from real people who actually wanted to talk to him.
Re:I call bullshit! - You got me! (Score:2)
You were almost right, but it was my abusive step-dad to tell me to come out of my attic room and clean up his beer cans, my online friend from Japan inviting me to come to Tokyo for a Manga festival, and the Thai delivery service confirming my order of
Tantrums (Score:4, Funny)
Re:Tantrums (Score:3, Informative)
Re:Tantrums (Score:2)
No, that's reserved for ridiculously ludicrous speculation that ultimately turns out to be true.
Did you hear that? (Score:2, Insightful)
It will be interesting to see what weapon the BSD crowd will retaliate with.
Re:Did you hear that? (Score:3, Insightful)
I would just prefer that their response is to release a stable system using their method.
Re:Did you hear that? (Score:2, Insightful)
Re:Did you hear that? (Score:2, Funny)
My guesses are they will respond something like this:
FreeBSD: FreeBSD users will continue their campaign of random acts of elitist snobbery against Linux users.
OpenBSD: Theo will threaten to stop work on OpenSSH unless Linus gives him $10,000 for every nasty email he sends a *BSD developer.
NetBSD: Will stop developing the NetBSD port for Linus' microwave.
Re:Did you hear that? (Score:2)
"Oh Reginold! I disagree!!!"
Given the respective quality of the Linux and *BSD (Score:5, Funny)
Just finished reading "Thud!", sorry... (Score:2, Funny)
it goes "incompetent idiots."
It is a Torvalds.
That is not my COW.
Kernel Brawl (Score:2)
If I only had a brain (Score:2, Funny)
Yeah, that's a bad idea. It's been tried. (Score:3, Informative)
The basic idea is to fake some memory to memory copying operations by using the virtual memory hardware. More specificially, the idea is that when you do a big "write", the space just written becomes read-only to the writing process, rather than being actually copied. When the write is complete, read-only mode is turned off. This eliminates one copy.
The trouble with this is that when you manipulate the page table to do that, you have to do some cache invalidation. That usually results in cache misses, which outweigh the cost of the copy. So this usually is a lose. Linus points out that it looks good on benchmarks, because benchmarks typically aren't using data for anything and thus don't experience the cache misses.
Actually, copying is a relatively cheap operation in modern CPUs unless the copy is huge, since most of the work is done in the caches. The mania for "zero copy" complicates systems considerably, makes them less reliable, and, in the end, usually doesn't speed up real work by much.
Some of this mania comes from Microsoft FUD. At one time, Microsoft was claiming that an "enterprise OS" must be able to serve web pages from inside the kernel. This led to more Linux interest in "zero copy" approaches to be "competitive".
Re:Yeah, that's a bad idea. It's been tried. (Score:3, Insightful)
The basic idea is to fake some memory to memory copying operations by using the virtual memory hardware. More specificially, the idea is that when you do a big "write", the space just written becomes read-only to the writing process, rather than being actually copied. When the write is complete, read-only mode is turned off. This eliminates one copy.
The way CoW works is that when a process copies something
Re:Yeah, that's a bad idea. It's been tried. (Score:3, Informative)
Re:Yeah, that's a bad idea. It's been tried. (Score:3, Interesting)
I totally agree with that, I just go one step further and say that Torvalds is also a total idiot: VM games are bad, so use copying instead because that's less bad. But copying is also bad, so why it at all? Neither are good solutions.
The problem is that linux and bsd are using "virtual memor
Linus, my advice to you is simple... (Score:4, Funny)
gosh (Score:2)
linus is just a guy, quite opinionated, and quite harsh in his words at times. are we gonna see this kind of slashdot story everytime he misbehaves somewhat? (i hope not)
Just what we need..... (Score:2)
I dig Linus, he's a smart, capable and funny guy, but this kinda dissapointed me. He's better than that.
Sweet (Score:5, Funny)
I'll start.
BSD user: Linux is a confusing mess of programs and is less stable than BSD.
Linux user: Your still here? I thought you were dead by now?
Re:Sweet (Score:2)
Re:Sweet (Score:3, Interesting)
Linux: Where do you want to be tomorrow?
BSD: Are you guys coming or what?!
Fess up (Score:2)
Or did someone dress his plush tux up with an "I love Windows" sign and BSD daemon horns?
Linus sometimes calls people idiots (Score:5, Insightful)
Grass is green;
Oil is overpriced;
Absolute power corrupts absolutely.
GEEK WARS BEGIN!!! NEWS AT 11!! (Score:4, Funny)
In the words of George Takei "Hoooooooly geeeez!" This is news??
Just say what you mean (Score:3, Insightful)
Linus, who's becoming more outspoken as he ages, needs to find that line between anonymous forum geek and software spokesperson...and then not cross it. Calling anyone an incompetent idiot is both non-constructive if you're hoping to improve a situation, and just plain unfriendly in an area where cooperation amongst developers is so crucial (open source).
Obligatory Simpsons reference (Score:3, Funny)
The best thing about being a professional . (Score:3, Interesting)
I used to own restourant and also an Office supplies shop . It was quite interesting and made me some money , but I hated the fact that the most important factor in my life was pleasing(customers) or fighting(suppliers) other people . I had to constantly think what to say and how to behave .
I am no longer a business owner , and now I work with a rather gifted bunch of engineers , and frankly it gives me great pleasure to know that neither I nor the people I work with dont really care about being polite , clean shaven well spoken or good looking . I can be rude if I want to , they can be rude if they want to , and we all get along very well .
Lesson on Tact.. (Score:3, Insightful)
Being nice and respectful doesn't mean you can't tell it like it is.
He has the stones to back it up (Score:4, Insightful)
How is this different than AIO? (Score:3, Informative)
For example, aio_write() writes to the file descriptor, allows you to poll for success a la select, and tells you not to modify the buffer before it's done (but doesn't try to stop you with copy-on-write).
This sounds exactly like what Linus wants.
An explanation (Score:5, Informative)
There seem to be a LOT of misconceptions about the discussion of vmslice() vs COW vs copy. This has nothing to do with conserving memory and everything to do with high performance I/O. If your app just needs to send a couple small files from A to B, you probably don't care about this at all.
A little background is needed on the terminology and mechanisms of I/O for any of this to make sense. For an example, let's say your app is a very busy web server sending dynamic (but trivial to compute) pages out.
The oldest and simplest method is copy. The app calls write(int sock, char *buffer, int length) on a socket. The kernel coppies the contents of buffer from userspace memory into a kernel space buffer and at least queues the data to the TCP stack before returning.
COW is an attempt to avoid the cost of copying the outgoing data.. In that case, the reference count on the physical pages that make up buffer is bumped up (since now kernel and application are both interested in them), and marks the pages as COW. That is, the virtual memory addresses are set as read only and a flag bit is set (more or less). The latter is done so the kernel needn't worry about them again. By the time the write call returns, the app is able to immediatly write to that memory (sorta) without worry.
When that write happens, the app takes a page fault (writing to a read-only page). The kernel sees that the pages are COW, copies the data to a new physical page, and maps the page in read/write. Then it returns from the fault. OTOH, if the kernel finished with the page first (the data goes on to the wire), it re-marks the page(s) so the app can access them without a copy.
The hope is that often enough, the app WON'T try to write to the pages while they're busy and so the cost of that copy is saved. If that hope comes through often enough it MIGHT be vaguely uesful. I say MIGHT since there is a significant cost just for marking the pages (the CPU's TLB must be flushed for the change to take effect). If the faults happen, it's a BIG loss since handling a fault takes thousands of CPU cycles.
So, for it to have any chance to help, the application programmer must already know enough to TRY to avoid writing to the same buffer again until it gets to the wire. Unfortunatly, it can never be sure so most apps don't bother.
The vmsplice() proposal is fairly simple. In this case, the app explicitly requests special treatment of the write. The pages are NOT marked as read only at all. Instead, the app is on it's honor to leave them alone until the kernel notifies it that they are again available. This saves the copy and the costs of TLB flush AND the (potential) cost of page faults. If the app breaks it's promise, it is the only one to suffer as the data it sent is corrupted (no kernel housekeeping is ever stored in such pages so there are no security implications). Any damage the app might do by sending screwy data could also be done using the old copy method.
What it all comes down to is that playing tricks with page mapping LOOKS nice at first glance since it SEEMS reasonable that not copying bytes around will save CPU cycles and memory bandwidth. The re-mapping (or just permission changes) on pages SEEMS lightweight. Unfortunatly, in fact, re-mapping or changing permission forces cache invalidations and page faults are just plain expensive. With the direction CPU design is going, these things will likely get more expensive rather than less (as they have for most of the history of microprocessor design).
It's really not that complex for an application to use. At least in comparison to the complexities and level of knowledge required to write an app that performs well enough to need this in the first place.
Re:Linus needs a cup of 'nice' (Score:2)
Re:Chiiiiiiil. (Score:2)
Re:Chiiiiiiil. (Score:2)
According to some, like Java and C# fanboys for example, just about anyone who programs still using C...
The Universe In Which Spock Has A Beard? (Score:5, Funny)
The Universe In Which Spock Has A Beard?
-- Terry
Re:Linus is turning into a dictator (Score:2, Informative)
RMS, ever heard of him?
He's just a developer (Score:3, Insightful)
As far as this whole VM thing goes, time and testing will tell the true story. Meanwhile, maybe we c
Re:Linus is turning into a dictator (Score:4, Insightful)
Dictator? Are the FreeBSD developers somehow unable to keep their implementation now that Linus deems it stupid?
You might feel he's being a bit of an arsehole, but that doesn't mean he's a dictator. He's not stopping anybody from doing anything, he's merely sharing his opinion of a development technique on a mailing list dedicated to discussing the development of his kernel.
Re:Linus is turning into a dictator (Score:5, Insightful)
the BSD guys have their reasoning and if you read more info about this it is not a shot in the dark that Linus is taking but he is frustrated that after many discussions nobody cares as much as he does on the performance issues.
Go back and read what Linus did back in the early days, it's no different today than what it was in 1990, he will call a duck a duck.
Re:Linus is turning into a dictator (Score:5, Insightful)
You'll forgive me for taking that with a grain of salt so long as memory over-commit remains the default mode of operation within Linux.
Re:Linus is turning into a dictator (Score:3, Insightful)
Re:Hmm, where have I heard this recently? (Score:3, Insightful)
No. Thats why there is more than one BSD. Issues come up, and booom crash goes the fork. Pity.
Re:Maybe Linus needs a vacation? (Score:2)