Re: Direct I/O performance problems with 1GB pages

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe