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    SubjectRe: [PATCH RT 00/02] SLOB optimizations
    On Tue, 2005-12-20 at 13:43 -0600, Matt Mackall wrote:
    > >
    > > I bet after a while of running, your performance will still suffer due
    > > to fragmentation. The more fragmented it is, the more space you lose
    > > and the more steps you need to walk.
    > >
    > > Remember, because of the small stack, kmalloc and kfree are used an
    > > awful lot. And if you slow those down, you will start to take a big hit
    > > in performance.
    > True, with the exception that the improved packing may be the
    > difference between fitting the working set in memory and
    > thrashing/OOMing for some applications. Not running at all =
    > infinitely bad performance.

    Well the best way to see, is to try it out with real applications on
    small machines. I guess I need to pull out my IBM Thinkpad 75c (32
    megs, I'll need to only allocate half) and try out the two and see how
    far I can push it. Unfortunately, this test may need to wait, since I
    have a ton of other things to push out first.

    If someone else (perhaps yourself) would like to give my patches a try,
    I would be really appreciate it. :)

    > And the fragmentation is really not all that bad. Remember, Linux and
    > other legacy systems used similar allocators for ages.

    But the performance, was greatly reduced, and the system just booted up.

    > > Ingo can answer this better himself, but I have a feeling he jumped to
    > > your SLOB system just because of the simplicity.
    > And only a config switch away..
    > > > This I like a lot. I'd like to see a size/performance measurement of
    > > > this by itself. I suspect it's an unambiguous win in both categories.
    > >
    > > Actually the performance gain was disappointingly small. As it was a
    > > separate patch and I though it would gain a lot. But if IIRC, it only
    > > increased the speed by a second or two (of the 1 minute 27 seconds).
    > > That's why I spent so much time in the next approach.
    > Still, if it's a size win, it definitely makes sense to merge.
    > Removing the big block list lock is also a good thing and might make a
    > bigger difference on SMP.

    Well, I guess I can check out the -mm branch and at least port the first
    patch over.

    > > > > The next patch was the big improvement, with the largest changes. I
    > > > > took advantage of how the kmem_cache usage that SLAB also takes
    > > > > advantage of. I created a memory pool like the global one, but for
    > > > > every cache with a size less then PAGE_SIZE >> 1.
    > > >
    > > > Hmm. By every size, I assume you mean powers of two. Which negates
    > > > some of the fine-grained allocation savings that current SLOB provides.
    > >
    > > Yeah its the same as what the slabs use. But I would like to take
    > > measurements of a running system between the two approaches. After a
    > > day of heavy network traffic, see what the fragmentation is like and how
    > > much is wasted. This would require me finishing my cache_chain work,
    > > and adding something similar to your SLOB.
    > >
    > > But the powers of two is only for the kmalloc, which this is a know
    > > behavior of the current system. So it <should> only be used for things
    > > that would alloc and free within a quick time (like for things you would
    > > like to put on a stack but cant), or the size is close to (less than or
    > > equal) a power of two. Otherwise a kmem_cache is made which is the size
    > > of expected object (off by UNIT_SIZE).
    > There are a fair number of long-lived kmalloc objects. You might try
    > playing with the kmalloc accounting patch in -tiny to see what's out
    > there.

    I'll have to try this out too. Thanks for the link.
    > > Oh, this reminds me, I probably still need to add a shrink cache
    > > algorithm. Which would be very hard to do in the current SLOB.
    > Hmmm? It already has one.

    The current version in Ingo's 2.6.15-rc5-rt2 didn't have one.

    > > > For what it's worth, I think we really ought to consider a generalized
    > > > allocator approach like Sun's VMEM, with various removable pieces.
    > >
    > > Interesting, I don't know how Sun's VMEM works. Do you have links to
    > > some documentation?

    Thanks, I'll read up on this.

    > > That looks like quite an undertaking, but may be well worth it. I think
    > > Linux's memory management is starting to show it's age. It's been
    > > through a few transformations, and maybe it's time to go through
    > > another. The work being done by the NUMA folks, should be taking into
    > > account, and maybe we can come up with a way that can make things easier
    > > and less complex without losing performance.
    > Fortunately, it can be done completely piecemeal.

    If you would like me to test any code, I'd be happy to when I have time.
    And maybe even add a few patches myself.

    -- Steve

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