lkml.org 
[lkml]   [2018]   [Mar]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock
    On Thu 01-03-18 14:28:45, Aaron Lu wrote:
    > When a page is freed back to the global pool, its buddy will be checked
    > to see if it's possible to do a merge. This requires accessing buddy's
    > page structure and that access could take a long time if it's cache cold.
    >
    > This patch adds a prefetch to the to-be-freed page's buddy outside of
    > zone->lock in hope of accessing buddy's page structure later under
    > zone->lock will be faster. Since we *always* do buddy merging and check
    > an order-0 page's buddy to try to merge it when it goes into the main
    > allocator, the cacheline will always come in, i.e. the prefetched data
    > will never be unused.
    >
    > In the meantime, there are two concerns:
    > 1 the prefetch could potentially evict existing cachelines, especially
    > for L1D cache since it is not huge;
    > 2 there is some additional instruction overhead, namely calculating
    > buddy pfn twice.
    >
    > For 1, it's hard to say, this microbenchmark though shows good result but
    > the actual benefit of this patch will be workload/CPU dependant;
    > For 2, since the calculation is a XOR on two local variables, it's expected
    > in many cases that cycles spent will be offset by reduced memory latency
    > later. This is especially true for NUMA machines where multiple CPUs are
    > contending on zone->lock and the most time consuming part under zone->lock
    > is the wait of 'struct page' cacheline of the to-be-freed pages and their
    > buddies.
    >
    > Test with will-it-scale/page_fault1 full load:
    >
    > kernel Broadwell(2S) Skylake(2S) Broadwell(4S) Skylake(4S)
    > v4.16-rc2+ 9034215 7971818 13667135 15677465
    > patch2/3 9536374 +5.6% 8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
    > this patch 10338868 +8.4% 8544477 +2.8% 14839808 +5.5% 17155464 +2.9%
    > Note: this patch's performance improvement percent is against patch2/3.

    I am really surprised that this has such a big impact. Is this a win on
    other architectures as well?

    > [changelog stole from Dave Hansen and Mel Gorman's comments]
    > https://lkml.org/lkml/2018/1/24/551

    Please use http://lkml.kernel.org/r/<msg-id> for references because
    lkml.org is quite unstable. It would be
    http://lkml.kernel.org/r/148a42d8-8306-2f2f-7f7c-86bc118f8ccd@intel.com
    here.
    --
    Michal Hocko
    SUSE Labs

    \
     
     \ /
      Last update: 2018-03-01 15:01    [W:2.348 / U:1.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site