linux-kernel - Re: [PATCH RFC V4 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once

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Date:	Sun, 30 Aug 2015 08:52:09 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	davem@...emloft.net, kuznet@....inr.ac.ru, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net, jiri@...nulli.us,
	edumazet@...gle.com, hannes@...essinduktion.org,
	tom@...bertland.com, azhou@...ira.com, ebiederm@...ssion.com,
	ipm@...rality.org.uk, nicolas.dichtel@...nd.com,
	serge.hallyn@...onical.com, joe@...ches.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	anton@....ibm.com, nacc@...ux.vnet.ibm.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH RFC V4 2/2] net: Optimize snmp stat aggregation by
 walking all the percpu data at once

On Sun, 2015-08-30 at 11:29 +0530, Raghavendra K T wrote:
> Docker container creation linearly increased from around 1.6 sec to 7.5 sec
> (at 1000 containers) and perf data showed 50% ovehead in snmp_fold_field.
> 
> reason: currently __snmp6_fill_stats64 calls snmp_fold_field that walks
> through per cpu data of an item (iteratively for around 36 items).
> 
> idea: This patch tries to aggregate the statistics by going through
> all the items of each cpu sequentially which is reducing cache
> misses.
> 
> Docker creation got faster by more than 2x after the patch.
> 
> Result:
>                        Before           After
> Docker creation time   6.836s           3.25s
> cache miss             2.7%             1.41%
> 
> perf before:
>     50.73%  docker           [kernel.kallsyms]       [k] snmp_fold_field
>      9.07%  swapper          [kernel.kallsyms]       [k] snooze_loop
>      3.49%  docker           [kernel.kallsyms]       [k] veth_stats_one
>      2.85%  swapper          [kernel.kallsyms]       [k] _raw_spin_lock
> 
> perf after:
>     10.57%  docker           docker                [.] scanblock
>      8.37%  swapper          [kernel.kallsyms]     [k] snooze_loop
>      6.91%  docker           [kernel.kallsyms]     [k] snmp_get_cpu_field
>      6.67%  docker           [kernel.kallsyms]     [k] veth_stats_one
> 
> changes/ideas suggested:
> Using buffer in stack (Eric), Usage of memset (David), Using memcpy in
> place of unaligned_put (Joe).
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> ---

Acked-by: Eric Dumazet <edumazet@...gle.com>

Thanks.



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