linux-kernel - Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and RUN_TO

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Message-ID: <3f24c6a9-27d7-46ad-a479-18a806444740@arm.com>
Date: Tue, 19 Nov 2024 11:29:07 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Joseph Salisbury <joseph.salisbury@...cle.com>,
 Cristian Prundeanu <cpru@...zon.com>, linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
 linux-arm-kernel@...ts.infradead.org, Bjoern Doebel <doebel@...zon.com>,
 Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
 Geoff Blake <blakgeof@...zon.com>, Ali Saidi <alisaidi@...zon.com>,
 Csaba Csoma <csabac@...zon.com>
Subject: Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and
 RUN_TO_PARITY and move them to sysctl

On 14/11/2024 21:10, Joseph Salisbury wrote:

Hi Joseph,

> On 10/17/24 01:19, Cristian Prundeanu wrote:

[...]

> Hi Cristian,
> 
> This is a confirmation that we are also seeing a 9% performance
> regression with the TPCC benchmark after v6.6-rc1.  We narrowed down the
> regression was caused due to commit:
> 86bfbb7ce4f6 ("sched/fair: Add lag based placement")
> 
> This regression was reported via this thread:
> https://lore.kernel.org/lkml/1c447727-92ed-416c-bca1-a7ca0974f0df@oracle.com/
> 
> Phil Auld suggested to try turning off the PLACE_LAG sched feature. We
> tested with NO_PLACE_LAG and can confirm it brought back 5% of the
> performance loss.  We do not yet know what effect NO_PLACE_LAG will have
> on other benchmarks, but it indeed helps TPCC.

Can you try to run mysql in SCHED_BATCH when using EEVDF?

https://lkml.kernel.org/r/20241029045749.37257-1-cpru@amazon.com

The regression went away for me when changing mysql threads to SCHED_BATCH.

You can either start mysql with 'CPUSchedulingPolicy=batch':

#cat /etc/systemd/system/mysql.service

[Service]
CPUSchedulingPolicy=batch
ExecStart=/usr/local/mysql/bin/mysqld_safe

# systemctl daemon-reload
# systemctl restart mysql

or change the policy with chrt for all mysql threads when doing
consecutive test runs starting from the 2. run ('connection' threads
have to exists already)

# chrt -b -a -p 0 $PID_MYSQL

# ps -p $PID_MYSQL -To comm,pid,tid,nice,class

COMMAND             PID     TID  NI CLS
mysqld             4872    4872   0 B
ib_io_ibuf         4872    4878   0 B
...
xpl_accept-3       4872    4921   0 B
connection         4872    5007   0 B
...
connection         4872    5413   0 B

My hunch is that this is due to the 'connection' threads (1 per virtual
user) running in SCHED_BATCH. I yet have to confirm this by only
changing the 'connection' tasks to SCHED_BATCH.


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