On 27.01.25 19:54, Jens Axboe wrote:
On 1/27/25 11:21 AM, Andres Freund wrote:
That's precisely what io-uring fixed buffers do :)
I looked at using them at some point - unfortunately it seems that there is
just {READ,WRITE}_FIXED not {READV,WRITEV}_FIXED. It's *exceedingly* common
for us to do reads/writes where source/target buffers aren't wholly
contiguous. Thus - unless I am misunderstanding something, entirely plausible
- using fixed buffers would unfortunately increase the number of IOs
noticeably.
Should have sent an email about that...
I guess we could add some heuristic to use _FIXED if it doesn't require
splitting an IO into too many sub-ios. But that seems pretty gnarly.
Adding Pavel, he's been working on support registered buffers with
readv/writev.
I dimly recall that I also ran into some around using fixed buffers as a
non-root user. It might just be the accounting of registered buffers as
mlocked memory and the difficulty of configuring that across
distributions. But I unfortunately don't remember any details anymore.
Should just be accounting.
For hugetlb we might be able to relax the accounting restriction. It's
effectively mlocked already, and setting up hugetlb requires privileges.
--
Cheers,
David / dhildenb
