Re: tbench regression with 2.6.32-rc1

Peter Zijlstra
2009-10-09T10:17:23Z

On Fri, 2009-10-09 at 17:51 +0800, Zhang, Yanmin wrote:
> Comparing with 2.6.31's results, tebench has some regression with
> 2.6.32-rc1.
> COmmandline to start tbench:
> #./tbench_srv &
> #./tbench -t 600 CPU_NUM*2 127.0.0.1 #Use real cpu num to replace CPU_NUM
> So start 2 client processes per cpu.
>
> 1) On 4*4 core tigerton: 30%;
> 2) On 2*4 core stoakley: 15%;
> 3) On 2*8 core Nehalem: 6%.
>
> As there are couple of patches which try to turn on/off some sched domain
> flags such like SD_BALANCE_WAKE, I used some walkaround to bisect it.
> On tigerton, below patch is captured.
> commit 59abf02644c45f1591e1374ee7bb45dc757fcb88
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Wed Sep 16 08:28:30 2009 +0200
>
> sched: Add SD_PREFER_LOCAL
>
>
> The patch reverting is not clean, so I did some testing by turning on/off
> some domain flags and sched_feaures manually.
>
> 1) On tigerton: if SD_PREFER_LOCAL=0 (disable it), the regression becomes about 2%.
> 2) On stoakley: if ﻿SD_PREFER_LOCAL=0 (disable it), the regression becomes about 4%.
> 3) On Nehalem: Above method couldn't improve result. I'm still checking it.
>
> I also tried to turn on/off﻿ FAIR_SLEEPERS and GENTLE_FAIR_SLEEPERS. It seems they
> has limited impact on tbench. I need double check these 2 flags.

So the c2q cpus, and esp the one with smaller cache hurt from this. I
guess we can turn this off without too much down sides. Maybe turn it on
for NUMA on the nehalem?

---
diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index 25a9284..d823c24 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -143,6 +143,7 @@ extern unsigned long node_remap_size[];
| 1*SD_BALANCE_FORK \
| 0*SD_BALANCE_WAKE \
| 1*SD_WAKE_AFFINE \
+ | 1*SD_PREFER_LOCAL \
| 0*SD_SHARE_CPUPOWER \
| 0*SD_POWERSAVINGS_BALANCE \
| 0*SD_SHARE_PKG_RESOURCES \
diff --git a/include/linux/topology.h b/include/linux/topology.h
index fc0bf3e..57e6357 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -129,7 +129,7 @@ int arch_update_cpu_topology(void);
| 1*SD_BALANCE_FORK \
| 0*SD_BALANCE_WAKE \
| 1*SD_WAKE_AFFINE \
- | 1*SD_PREFER_LOCAL \
+ | 0*SD_PREFER_LOCAL \
| 0*SD_SHARE_CPUPOWER \
| 1*SD_SHARE_PKG_RESOURCES \
| 0*SD_SERIALIZE \
@@ -162,7 +162,7 @@ int arch_update_cpu_topology(void);
| 1*SD_BALANCE_FORK \
| 0*SD_BALANCE_WAKE \
| 1*SD_WAKE_AFFINE \
- | 1*SD_PREFER_LOCAL \
+ | 0*SD_PREFER_LOCAL \
| 0*SD_SHARE_CPUPOWER \
| 0*SD_SHARE_PKG_RESOURCES \
| 0*SD_SERIALIZE \