Re: [patch 00/18] CFS Bandwidth Control v7.2

Paul Turner
2011-07-21T23:02:09Z

On Thu, Jul 21, 2011 at 9:43 AM, Paul Turner <pjt@google.com> wrote:
> Hi all,
>
> Please find attached the incremental v7.2 for bandwidth control.
>
> This release follows a fairly intensive period of scraping cycles across
> various configurations.  Unfortunately we seem to be currently taking an IPC
> hit for jump_labels (despite a savings in branches/instr. ret) which despite
> fairly extensive digging I don't have a good explanation for.  The emitted
> assembly /looks/ ok, but cycles/wall time is consistently higher across several
> platforms.
>
> As such I've demoted the jumppatch to [RFT] while these details are worked
> out.  But there's no point in holding up the rest of the series any more.
>
> [ Please find the specific discussion related to the above attached to patch
> 17/18. ]
>
> So -- without jump labels -- the current performance looks like:
>
>                            instructions            cycles                  branches
> ---------------------------------------------------------------------------------------------
> clovertown [!BWC]           843695716               965744453               151224759
> +unconstrained              845934117 (+0.27)       974222228 (+0.88)       152715407 (+0.99)
> +10000000000/1000:          855102086 (+1.35)       978728348 (+1.34)       154495984 (+2.16)
> +10000000000/1000000:       853981660 (+1.22)       976344561 (+1.10)       154287243 (+2.03)
>
> barcelona [!BWC]            810514902               761071312               145351489
> +unconstrained              820573353 (+1.24)       748178486 (-1.69)       148161233 (+1.93)
> +10000000000/1000:          827963132 (+2.15)       757829815 (-0.43)       149611950 (+2.93)
> +10000000000/1000000:       827701516 (+2.12)       753575001 (-0.98)       149568284 (+2.90)
>
> westmere [!BWC]             792513879               702882443               143267136
> +unconstrained              802533191 (+1.26)       694415157 (-1.20)       146071233 (+1.96)
> +10000000000/1000:          809861594 (+2.19)       701781996 (-0.16)       147520953 (+2.97)
> +10000000000/1000000:       809752541 (+2.18)       705278419 (+0.34)       147502154 (+2.96)
>
> Under the workload:
>  mkdir -p /cgroup/cpu/test
>  echo $$ > /dev/cgroup/cpu/test (only cpu,cpuacct mounted)
>  (W1) taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "for ((i=0;i<5;i++)); do $(dirname $0)/pipe-test 20000; done"
>
> This may seem a strange work-load but it works around some bizarro overheads
> currently introduced by perf.  Comparing for example with::w
>  (W2)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;true"
>  (W3)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;"
>
>
> We see:

(Sorry this is missing an "instructions,cycles,branches,elapsed time" header.)

>  (W1)  westmere [!BWC]             792513879               702882443               143267136             0.197246943
>  (W2)  westmere [!BWC]             912241728               772576786               165734252             0.214923134
>  (W3)  westmere [!BWC]             904349725               882084726               162577399             0.748506065
>
> vs an 'ideal' total exec time of (approximately):
> $ time taskset -c 0 ./pipe-test 100000
>  real    0m0.198 user    0m0.007s ys     0m0.095s
>
> The overhead in W2 is explained by that invoking pipe-test directly, one of
> the siblings is becoming the perf_ctx parent, invoking lots of pain every time
> we switch.  I do not have a reasonable explantion as to why (W1) is so much
> cheaper than (W2), I stumbled across it by accident when I was trying some
> combinations to reduce the <perf stat>-to-<perf stat> variance.
>
> v7.2
> -----------
> - Build errors in !CGROUP_SCHED case fixed
> - !CONFIG_SMP now 'supported' (#ifdef munging)
> - gcc was failing to inline account_cfs_rq_runtime, affecting performance
> - checks in expire_cfs_rq_runtime() and check_enqueue_throttle() re-organized
>  to save branches.
> - jump labels introduced in the case BWC is not being used system-wide to
>  reduce inert overhead.
> - branch saved in expiring runtime (reorganize conditonals)
>
> Hidetoshi, the following patchsets have changed enough to necessitate tweaking
> of your Reviewed-by:
> [patch 09/18] sched: add support for unthrottling group entities (extensive)
> [patch 11/18] sched: prevent interactions with throttled entities (update_cfs_shares)
> [patch 12/18] sched: prevent buddy interactions with throttled entities (new)
>
>
> Previous postings:
> -----------------
> v7.1: https://lkml.org/lkml/2011/7/7/24
> v7: http://lkml.org/lkml/2011/6/21/43
> v6: http://lkml.org/lkml/2011/5/7/37
> v5: http://lkml.org/lkml/2011/3 /22/477
> v4: http://lkml.org/lkml/2011/2/23/44
> v3: http://lkml.org/lkml/2010/10/12/44
> v2: http://lkml.org/lkml/2010/4/28/88
> Original posting: http://lkml.org/lkml/2010/2/12/393
>
> Prior approaches: http://lkml.org/lkml/2010/1/5/44 ["CFS Hard limits v5"]
>
> Thanks,
>
> - Paul
>
>