Re: [PATCH -V3 4/9] mm: restrict the pcp batch scale factor to avoid too long latency

Mel Gorman
2023-10-19T12:13:08Z

On Mon, Oct 16, 2023 at 01:29:57PM +0800, Huang Ying wrote:
> In page allocator, PCP (Per-CPU Pageset) is refilled and drained in
> batches to increase page allocation throughput, reduce page
> allocation/freeing latency per page, and reduce zone lock contention.
> But too large batch size will cause too long maximal
> allocation/freeing latency, which may punish arbitrary users. So the
> default batch size is chosen carefully (in zone_batchsize(), the value
> is 63 for zone > 1GB) to avoid that.
>
> In commit 3b12e7e97938 ("mm/page_alloc: scale the number of pages that
> are batch freed"), the batch size will be scaled for large number of
> page freeing to improve page freeing performance and reduce zone lock
> contention. Similar optimization can be used for large number of
> pages allocation too.
>
> To find out a suitable max batch scale factor (that is, max effective
> batch size), some tests and measurement on some machines were done as
> follows.
>
> A set of debug patches are implemented as follows,
>
> - Set PCP high to be 2 * batch to reduce the effect of PCP high
>
> - Disable free batch size scaling to get the raw performance.
>
> - The code with zone lock held is extracted from rmqueue_bulk() and
> free_pcppages_bulk() to 2 separate functions to make it easy to
> measure the function run time with ftrace function_graph tracer.
>
> - The batch size is hard coded to be 63 (default), 127, 255, 511,
> 1023, 2047, 4095.
>
> Then will-it-scale/page_fault1 is used to generate the page
> allocation/freeing workload. The page allocation/freeing throughput
> (page/s) is measured via will-it-scale. The page allocation/freeing
> average latency (alloc/free latency avg, in us) and allocation/freeing
> latency at 99 percentile (alloc/free latency 99%, in us) are measured
> with ftrace function_graph tracer.
>
> The test results are as follows,
>
> Sapphire Rapids Server
> ======================
> Batch throughput free latency free latency alloc latency alloc latency
> page/s avg / us 99% / us avg / us 99% / us
> ----- ---------- ------------ ------------ ------------- -------------
> 63 513633.4 2.33 3.57 2.67 6.83
> 127 517616.7 4.35 6.65 4.22 13.03
> 255 520822.8 8.29 13.32 7.52 25.24
> 511 524122.0 15.79 23.42 14.02 49.35
> 1023 525980.5 30.25 44.19 25.36 94.88
> 2047 526793.6 59.39 84.50 45.22 140.81
>
> Ice Lake Server
> ===============
> Batch throughput free latency free latency alloc latency alloc latency
> page/s avg / us 99% / us avg / us 99% / us
> ----- ---------- ------------ ------------ ------------- -------------
> 63 620210.3 2.21 3.68 2.02 4.35
> 127 627003.0 4.09 6.86 3.51 8.28
> 255 630777.5 7.70 13.50 6.17 15.97
> 511 633651.5 14.85 22.62 11.66 31.08
> 1023 637071.1 28.55 42.02 20.81 54.36
> 2047 638089.7 56.54 84.06 39.28 91.68
>
> Cascade Lake Server
> ===================
> Batch throughput free latency free latency alloc latency alloc latency
> page/s avg / us 99% / us avg / us 99% / us
> ----- ---------- ------------ ------------ ------------- -------------
> 63 404706.7 3.29 5.03 3.53 4.75
> 127 422475.2 6.12 9.09 6.36 8.76
> 255 411522.2 11.68 16.97 10.90 16.39
> 511 428124.1 22.54 31.28 19.86 32.25
> 1023 414718.4 43.39 62.52 40.00 66.33
> 2047 429848.7 86.64 120.34 71.14 106.08
>
> Commet Lake Desktop
> ===================
> Batch throughput free latency free latency alloc latency alloc latency
> page/s avg / us 99% / us avg / us 99% / us
> ----- ---------- ------------ ------------ ------------- -------------
>
> 63 795183.13 2.18 3.55 2.03 3.05
> 127 803067.85 3.91 6.56 3.85 5.52
> 255 812771.10 7.35 10.80 7.14 10.20
> 511 817723.48 14.17 27.54 13.43 30.31
> 1023 818870.19 27.72 40.10 27.89 46.28
>
> Coffee Lake Desktop
> ===================
> Batch throughput free latency free latency alloc latency alloc latency
> page/s avg / us 99% / us avg / us 99% / us
> ----- ---------- ------------ ------------ ------------- -------------
> 63 510542.8 3.13 4.40 2.48 3.43
> 127 514288.6 5.97 7.89 4.65 6.04
> 255 516889.7 11.86 15.58 8.96 12.55
> 511 519802.4 23.10 28.81 16.95 26.19
> 1023 520802.7 45.30 52.51 33.19 45.95
> 2047 519997.1 90.63 104.00 65.26 81.74
>
> From the above data, to restrict the allocation/freeing latency to be
> less than 100 us in most times, the max batch scale factor needs to be
> less than or equal to 5.
>
> Although it is reasonable to use 5 as max batch scale factor for the
> systems tested, there are also slower systems. Where smaller value
> should be used to constrain the page allocation/freeing latency.
>
> So, in this patch, a new kconfig option (PCP_BATCH_SCALE_MAX) is added
> to set the max batch scale factor. Whose default value is 5, and
> users can reduce it when necessary.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Acked-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

--
Mel Gorman
SUSE Labs