EEVDF regression still exists

[K Prateek Nayak]

Hello Cristian,

On 4/30/2025 3:36 AM, Prundeanu, Cristian wrote:
> On 2025-04-29, 16:57, "Peter Zijlstra" <peterz at infradead.org <mailto:peterz at infradead.org>> wrote:
> 
>>> Here are the latest results for the EEVDF impact on database workloads.
>>> The regression introduced in kernel 6.6 still persists and doesn't look
>>> like it is improving.
>>
>> Well, I was under the impression it had actually been solved :-(
>>
>> My understanding from the last round was that Prateek and co had it
>> sorted -- with the caveat being that you had to stick SCHED_BATCH in at
>> the right place in MySQL start scripts or somesuch.
> 
> The statement in the previous thread [1] was that using SCHED_BATCH improves
> performance over default. While that still holds true, it is also equally true
> about using SCHED_BATCH on kernel 6.5.
> 
> So, when we compare 6.5 with recent kernels, both using SCHED_BATCH, the
> regression is still visible. (Previously, we only compared SCHED_BATCH with
> 6.5 default, leading to the wrong conclusion that it's a fix).

So I never tried comparing SCHED_BATCH on both old vs new kernel for
the HammerDB benchmark since SCHED_BATCH had not led to a great
improvement in the baseline numbers on v6.5 in my previous debugs and
I was mostly looking at context-switch data, trying to match the EEVDF
case to baseline numbers.

I'll try to setup the reproducer you have posted on my end and reach
out if I run into any issues. Hopefully the exact setup reveals
something I've overlooked.

P.S. Are the numbers for v6.15-rc4 + SCHED_BATCH comparable to v6.5
default?

One more curious question: Does changing the base slice to a larger
value (say 6ms) in conjunction with setting SCHED_BATCH on v6.15-rc4
affect the benchmark result in any way?

> 
> [1] https://lore.kernel.org/all/feb31b6e-6457-454c-a4f3-ce8ad96bf8de@amd.com/
> 

-- 
Thanks and Regards,
Prateek