lkml.org 
[lkml]   [2014]   [Apr]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 2/6] x86: mm: rip out complicated, out-of-date, buggy TLB flushing
    On Thu, Apr 24, 2014 at 09:58:11AM -0700, Dave Hansen wrote:
    > On 04/24/2014 01:45 AM, Mel Gorman wrote:
    > >> +/*
    > >> + * See Documentation/x86/tlb.txt for details. We choose 33
    > >> + * because it is large enough to cover the vast majority (at
    > >> + * least 95%) of allocations, and is small enough that we are
    > >> + * confident it will not cause too much overhead. Each single
    > >> + * flush is about 100 cycles, so this caps the maximum overhead
    > >> + * at _about_ 3,000 cycles.
    > >> + */
    > >> +/* in units of pages */
    > >> +unsigned long tlb_single_page_flush_ceiling = 1;
    > >> +
    > >
    > > This comment is premature. The documentation file does not exist yet and
    > > 33 means nothing yet. Out of curiousity though, how confident are you
    > > that a TLB flush is generally 100 cycles across different generations
    > > and manufacturers of CPUs? I'm not suggesting you change it or auto-tune
    > > it, am just curious.
    >
    > Yeah, the comment belongs in the later patch where I set it to 33.
    >
    > I looked at this on the last few generations of Intel CPUs. "100
    > cycles" was a very general statement, and not precise at all. My laptop
    > averages out to 113 cycles overall, but the flushes of 25 pages averaged
    > 96 cycles/page while the flushes of 2 averaged 219/page.
    >
    > Those cycles include some costs of from the instrumentation as well.
    >
    > I did not test on other CPU manufacturers, but this should be pretty
    > easy to reproduce. I'm happy to help folks re-run it on other hardware.
    >
    > I also believe with the modalias stuff we've got in sysfs for the CPU
    > objects we can do this in the future with udev rules instead of
    > hard-coding it in the kernel.
    >

    You convinced me. Regardless of whether you move the comment or update
    the changelog;

    Acked-by: Mel Gorman <mgorman@suse.de>

    --
    Mel Gorman
    SUSE Labs


    \
     
     \ /
      Last update: 2014-04-24 20:21    [W:2.386 / U:1.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site