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Re: slow memory I/O on AMD EPYC 9334

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On Fri, 2025-10-17 at 14:11 +0200, Gero wrote:
> Dear experts,
> 
> thanks a lot for your work and commitment on the Debian system. I'm 
> using Debian for years and I am generally very pleased with it.  :-)
> 
> With my company we do numerical simulations and recently did some 
> benchmarking tests on new AMD EPYC 9334 processors that showed a 
> significant performance loss of a current Debian system compared to an 
> older Red Hat or Rocky Linux. We could narrow that down to the following 
> finding:
> 
> Running these commands:
> 
> cd /dev/shm
> echo 3 > /proc/sys/vm/drop_caches
> fio --name=random-write --ioengine=posixaio --rw=randwrite --bs=4k 
> --numjobs=1 --size=4g --iodepth=1 --runtime=60 --time_based --end_fsync=1
[...]
> I would like to know if you have an explanation or an idea.

I think for this specific test, the explanation is "this is a stupid I/O
pattern that no-one optimises for".  Using AIO with a depth of 1 is
effectively doing synchronous I/O in a less efficient way.

Added to that, POSIX AIO was never that efficient on Linux, and the
upstream developers seem to have more-or-less given up on it in favour
of io_uring.

> And I wonder
> if you would be interested in investigating the issue any further. Or if
> you have a suggestion who I might address preferably.

If you can also see a regression for io_uring and a more sensible I/O
depth then this would probably be interesting for the upstream
developers,

Ben.

-- 
Ben Hutchings
Computers are not intelligent.	They only think they are.

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Re: slow memory I/O on AMD EPYC 9334 Ben Hutchings <ben@decadent.org.uk> - 2025-10-20 19:30 +0200

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