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    SubjectRe: SLUB performance regression vs SLAB
    On Fri, Oct 05 2007, David Chinner wrote:
    > On Thu, Oct 04, 2007 at 03:07:18PM -0700, David Miller wrote:
    > > From: Chuck Ebbert <cebbert@redhat.com> Date: Thu, 04 Oct 2007 17:47:48
    > > -0400
    > >
    > > > On 10/04/2007 05:11 PM, David Miller wrote:
    > > > > From: Chuck Ebbert <cebbert@redhat.com> Date: Thu, 04 Oct 2007 17:02:17
    > > > > -0400
    > > > >
    > > > >> How do you simulate reading 100TB of data spread across 3000 disks,
    > > > >> selecting 10% of it using some criterion, then sorting and summarizing
    > > > >> the result?
    > > > >
    > > > > You repeatedly read zeros from a smaller disk into the same amount of
    > > > > memory, and sort that as if it were real data instead.
    > > >
    > > > You've just replaced 3000 concurrent streams of data with a single stream.
    > > > That won't test the memory allocator's ability to allocate memory to many
    > > > concurrent users very well.
    > >
    > > You've kindly removed my "thinking outside of the box" comment.
    > >
    > > The point is was not that my specific suggestion would be perfect, but that
    > > if you used your creativity and thought in similar directions you might find
    > > a way to do it.
    > >
    > > People are too narrow minded when it comes to these things, and that's the
    > > problem I want to address.
    >
    > And it's a good point, too, because often problems to one person are a
    > no-brainer to someone else.
    >
    > Creating lots of "fake" disks is trivial to do, IMO. Use loopback on
    > sparse files containing sparse filesxi, use ramdisks containing sparse
    > files or write a sparse dm target for sparse block device mapping,
    > etc. I'm sure there's more than the few I just threw out...

    Or use scsi_debug to fake drives/controllers, works wonderful as well
    for some things and involve the full IO stack.

    I'd like to second Davids emails here, this is a serious problem. Having
    a reproducible test case lowers the barrier for getting the problem
    fixed by orders of magnitude. It's the difference between the problem
    getting fixed in a day or two and it potentially lingering for months,
    because email ping-pong takes forever and "the test team has moved on to
    other tests, we'll let you know the results of test foo in 3 weeks time
    when we have a new slot on the box" just removing any developer
    motivation to work on the issue.

    --
    Jens Axboe

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