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

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Message-ID: <570e25e1-03c2-4ae1-8e4c-447a453a9d34@arm.com>
Date: Wed, 30 Oct 2024 11:21:06 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Cristian Prundeanu <cpru@...zon.com>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>
Cc: linux-tip-commits@...r.kernel.org, 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>,
 Benjamin Herrenschmidt <benh@...nel.crashing.org>,
 K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and
 RUN_TO_PARITY and move them to sysctl

Hi Christian,

On 29/10/2024 05:57, Cristian Prundeanu wrote:
> Hi Gautham,
> 
> On 2024-10-25, 09:44, "Gautham R. Shenoy" <gautham.shenoy@....com <mailto:gautham.shenoy@....com>> wrote:
> 
>> On Thu, Oct 24, 2024 at 07:12:49PM +1100, Benjamin Herrenschmidt wrote:
>>> On Sat, 2024-10-19 at 02:30 +0000, Prundeanu, Cristian wrote:
>>>>
>>>> The hammerdb test is a bit more complex than sysbench. It uses two
>>>> independent physical machines to perform a TPC-C derived test [1], aiming
>>>> to simulate a real-world database workload. The machines are allocated as
>>>> an AWS EC2 instance pair on the same cluster placement group [2], to avoid
>>>> measuring network bottlenecks instead of server performance. The SUT
>>>> instance runs mysql configured to use 2 worker threads per vCPU (32
>>>> total); the load generator instance runs hammerdb configured with 64
>>>> virtual users and 24 warehouses [3]. Each test consists of multiple
>>>> 20-minute rounds, run consecutively on multiple independent instance
>>>> pairs.
>>>
>>> Would it be possible to produce something that Prateek and Gautham
>>> (Hi Gautham btw !) can easily consume to reproduce ?
>>>
>>> Maybe a container image or a pair of container images hammering each
>>> other ? (the simpler the better).
>>
>> Yes, that would be useful. Please share your recipe. We will try and
>> reproduce it at our end. In our testing from a few months ago (some of
>> which was presented at OSPM 2024), most of the database related
>> regressions that we observed with EEVDF went away after running these
>> the server threads under SCHED_BATCH.
> 
> I am working on a repro package that is self contained and as simple to 
> share as possible.
> 
> My testing with SCHED_BATCH is meanwhile concluded. It did reduce the 
> regression to less than half - but only with WAKEUP_PREEMPTION enabled. 
> When using NO_WAKEUP_PREEMPTION, there was no performance change compared 
> to SCHED_OTHER.

Which tasks did you set SCHED_BATCH here? I'm assuming the mysql
'connection' tasks on the SUT (1 task for each virtual user I guess).

I did this and see that the regression goes away. I'm using a similar
test setup (hammerdb - mysql on AWS EC2 instances).

I'm not sure yet how reliable my results are. The big unknown is the
host system when I use AWS EC2 instances for hammerdb (Load Gen) and
mysql (server). In case I gather test results over multiple days, the
host system might have changed?

I also tried the (not-mainlined) RESPECT_SLICE (NO_RUN_TO_PARITY)
features which shows similar results compared to SCHED_BATCH for those
threads.

IIRC, RESPECT_SLICE was also helping Gautham to get the performance back
for his 'sysbench + mysql' workload:

OSPM 24 link to his presentation:

https://youtu.be/jrEN4pJiRWU?t=1115

> (At the risk of stating the obvious, using SCHED_BATCH only to get back to 
> the default CFS performance is still only a workaround, just as disabling 
> PLACE_LAG+RUN_TO_PARITY is; these give us more room to investigate the 
> root cause in EEVDF, but shouldn't be seen as viable alternate solutions.)
> 
> Do you have more detail on the database regressions you saw a few months 
> ago? What was the magnitude, and which workloads did it manifest on?
> 
> -Cristian
>