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    SubjectRe: [PATCH v2 0/3] staging: zcache: xcfmalloc support
    On 09/15/2011 06:17 PM, Dave Hansen wrote:

    > On Thu, 2011-09-15 at 14:24 -0500, Seth Jennings wrote:
    >> How would you suggest that I measure xcfmalloc performance on a "very
    >> large set of workloads". I guess another form of that question is: How
    >> did xvmalloc do this?
    > Well, it didn't have a competitor, so this probably wasn't done. :)

    A lot of testing was done for xvmalloc (and its predecessor, tlsf)
    before it was integrated into zram:

    I think we can use the same set of testing tools. See:

    These tools do issue mix of alloc and frees each with some probability
    which can be adjusted in code.

    There is also a tool called "swap replay" which collects swap-out traces
    and simulates the same behavior in userspace, allowing allocator testing
    with "real world" traces. See:

    > I'd like to see a microbenchmarky sort of thing. Do a million (or 100
    > million, whatever) allocations, and time it for both allocators doing
    > the same thing. You just need to do the *same* allocations for both.
    > It'd be interesting to see the shape of a graph if you did:
    > for (i = 0; i < BIG_NUMBER; i++)
    > for (j = MIN_ALLOC; j < MAX_ALLOC; j += BLOCK_SIZE)
    > alloc(j);
    > free();
    > ... basically for both allocators. Let's see how the graphs look. You
    > could do it a lot of different ways: alloc all, then free all, or alloc
    > one free one, etc... Maybe it will surprise us. Maybe the page
    > allocator overhead will dominate _everything_, and we won't even see the
    > x*malloc() functions show up.
    > The other thing that's important is to think of cases like I described
    > that would cause either allocator to do extra splits/joins or be slow in
    > other ways. I expect xcfmalloc() to be slowest when it is allocating
    > and has to break down a reserve page. Let's say it does a bunch of ~3kb
    > allocations and has no pages on the freelists, it will:
    > 1. scan each of the 64 freelists heads (512 bytes of cache)
    > 2. split a 4k page
    > 3. reinsert the 1k remainder
    > Next time, it will:
    > 1. scan, and find the 1k bit
    > 2. continue scanning, eventually touching each freelist...
    > 3. split a 4k page
    > 4. reinsert the 2k remainder
    > It'll end up doing a scan/split/reinsert in 3/4 of the cases, I think.
    > The case of the freelists being quite empty will also be quite common
    > during times the pool is expanding. I think xvmalloc() will have some
    > of the same problems, but let's see if it does in practice.


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