Re: [PATCH v2 0/3] Introduce SIS_CACHE to choose previous CPU during task wakeup

Madadi Vineeth Reddy
2023-11-26T08:44:58Z

Hi Chen Yu,

On 21/11/23 13:09, Chen Yu wrote:
> v1 -> v2:
> - Move the task sleep duration from sched_entity to task_struct. (Aaron Lu)
> - Refine the task sleep duration calculation based on task's previous running
> CPU. (Aaron Lu)
> - Limit the cache-hot idle CPU scan depth to reduce the time spend on
> searching, to fix the regression. (K Prateek Nayak)
> - Add test results of the real life workload per request from Ingo
> Daytrader on a power system. (Madadi Vineeth Reddy)
> OLTP workload on Xeon Sapphire Rapids.
> - Refined the commit log, added Reviewed-by tag to PATCH 1/3
> (Mathieu Desnoyers).
>
> RFC -> v1:
> - drop RFC
> - Only record the short sleeping time for each task, to better honor the
> burst sleeping tasks. (Mathieu Desnoyers)
> - Keep the forward movement monotonic for runqueue's cache-hot timeout value.
> (Mathieu Desnoyers, Aaron Lu)
> - Introduce a new helper function cache_hot_cpu() that considers
> rq->cache_hot_timeout. (Aaron Lu)
> - Add analysis of why inhibiting task migration could bring better throughput
> for some benchmarks. (Gautham R. Shenoy)
> - Choose the first cache-hot CPU, if all idle CPUs are cache-hot in
> select_idle_cpu(). To avoid possible task stacking on the waker's CPU.
> (K Prateek Nayak)
>
> Thanks for the comments and tests!
>
> ----------------------------------------------------------------------
>
> This series aims to continue the discussion of how to make the wakee
> to choose its previous CPU easier.
>
> When task p is woken up, the scheduler leverages select_idle_sibling()
> to find an idle CPU for it. p's previous CPU is usually a preference
> because it can improve cache locality. However in many cases, the
> previous CPU has already been taken by other wakees, thus p has to
> find another idle CPU.
>
> Inhibit the task migration could benefit many workloads. Inspired by
> Mathieu's proposal to limit the task migration ratio[1], introduce
> the SIS_CACHE. It considers the sleep time of the task for better
> task placement. Based on the task's short sleeping history, tag p's
> previous CPU as cache-hot. Later when p is woken up, it can choose
> its previous CPU in select_idle_sibling(). When other task is
> woken up, skip this cache-hot idle CPU and try the next idle CPU
> when possible. The idea of SIS_CACHE is to optimize the idle CPU
> scan sequence. The extra scan time is minimized by restricting the
> scan depth of cache-hot CPUs to 50% of the scan depth of SIS_UTIL.
>
> This test is based on tip/sched/core, on top of
> Commit ada87d23b734
> ("x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram")
>
> This patch set has shown 15% ~ 70% improvements for client/server
> workloads like netperf and tbench. It shows 0.7% improvement of
> OLTP with 0.2% run-to-run variation on Xeon 240 CPUs system.
> There is 2% improvement of another real life workload Daytrader
> per the test of Madadi on a power system with 96 CPUs. Prateek
> has helped check there is no obvious microbenchmark regression
> of the v2 on a 3rd Generation EPYC System with 128 CPUs.
>

Tested the patch on power system with 46 cores. Total of 368 CPU's.
System has 8 NUMA nodes.

Below are some of the benchmark results.

schbench(new) 99.0th latency (lower is better)
========
case load baseline[pct imp](std%) SIS_CACHE[pct imp]( std%)
normal 1-mthreads 1.00 [ 0.00]( 4.34) 1.02 [ -2.00]( 5.98)
normal 2-mthreads 1.00 [ 0.00]( 13.95) 1.08 [ -8.00]( 10.39)
normal 4-mthreads 1.00 [ 0.00]( 6.20) 0.94 [ +6.00]( 10.90)
normal 6-mthreads 1.00 [ 0.00]( 12.76) 1.03 [ -3.00]( 9.33)

It seems like schbench is not much impacted with this patch(The pct imp of schbench is within the std%).
I expected some regression in wakeup latency while searching for an idle cpu which is not cache hot.
But I guess limiting the search depth had helped.

producer_consumer avg time/access (lower is better)
========
loads per consumer iteration baseline[pct imp](std%) SIS_CACHE[pct imp]( std%)
5 1.00 [ 0.00]( 0.00) 0.93 [ +7.00]( 4.77)
10 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.00)
20 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.00)

The main goal of the patch of improving cache locality is reflected as SIS_CACHE only improves in this workload,
when loads per consumer iteration is lower.

hackbench normalized time in seconds (lower is better)
========
case load baseline[pct imp](std%) SIS_CACHE[pct imp]( std%)
process-sockets 1-groups 1.00 [ 0.00]( 4.78) 0.99 [ +1.00]( 6.45)
process-sockets 2-groups 1.00 [ 0.00]( 0.97) 1.02 [ -2.00]( 1.87)
process-sockets 4-groups 1.00 [ 0.00]( 3.63) 1.01 [ -1.00]( 2.96)
process-sockets 8-groups 1.00 [ 0.00]( 0.43) 1.00 [ 0.00]( 0.27)
process-pipe 1-groups 1.00 [ 0.00](23.77) 0.88 [+12.00](22.77)
process-pipe 2-groups 1.00 [ 0.00]( 3.44) 1.03 [ -3.00]( 4.00)
process-pipe 4-groups 1.00 [ 0.00]( 2.41) 0.98 [ +2.00]( 3.88)
process-pipe 8-groups 1.00 [ 0.00]( 7.09) 1.07 [ -7.00]( 4.25)
threads-pipe 1-groups 1.00 [ 0.00](18.47) 1.11 [-11.00](24.21)
threads-pipe 2-groups 1.00 [ 0.00]( 6.45) 0.97 [ +3.00]( 5.58)
threads-pipe 4-groups 1.00 [ 0.00]( 5.63) 0.96 [ +2.00]( 5.90)
threads-pipe 8-groups 1.00 [ 0.00]( 1.65) 1.03 [ -3.00]( 3.97)
threads-sockets 1-groups 1.00 [ 0.00]( 2.00) 1.00 [ 0.00]( 0.65)
threads-sockets 2-groups 1.00 [ 0.00]( 1.69) 1.02 [ -2.00]( 1.48)
threads-sockets 4-groups 1.00 [ 0.00]( 5.66) 1.01 [ -1.00]( 3.56)
threads-sockets 8-groups 1.00 [ 0.00]( 0.26) 0.99 [ +1.00]( 0.36)

hackbench is not impacted.

Daytrader throughput (higher is better)
========
instances,users baseline[pct imp](std%) SIS_CACHE[pct imp]( std%)
3,30 1.00 [ 0.00]( 2.30) 1.02 [ +2.00]( 1.64)
3,60 1.00 [ 0.00]( 0.55) 1.01 [ +1.00]( 1.41)
3,90 1.00 [ 0.00]( 1.20) 1.02 [ +2.00]( 1.04)
3,120 1.00 [ 0.00]( 0.84) 1.02 [ +2.00]( 1.02)

A real life workload like daytrader is benefiting slightly with this patch.

Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>

Thanks and Regards
Madadi Vineeth Reddy