Re: [PATCH 04/10] mm: restrict the pcp batch scale factor to avoid too long latency

[Mel Gorman]

On Wed, Sep 20, 2023 at 02:18:50PM +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.
> 
> So, in this patch, the batch scale factor is restricted to be less
> than or equal to 5.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

However, it's worth noting that the time to free depends on the CPU and
while the CPUs you tested are reasonable, there are also slower CPUs out
there and I've at least one account that the time is excessive. While
this patch is fine, there may be a patch on top that makes this runtime
configurable, a Kconfig default or both.

-- 
Mel Gorman
SUSE Labs