Re: Large CQE for fuse headers

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 10/11/24 19:57, Jens Axboe wrote:
> On 10/10/24 2:56 PM, Bernd Schubert wrote:
>> Hello,
>>
>> as discussed during LPC, we would like to have large CQE sizes, at least
>> 256B. Ideally 256B for fuse, but CQE512 might be a bit too much...
>>
>> Pavel said that this should be ok, but it would be better to have the CQE
>> size as function argument. 
>> Could you give me some hints how this should look like and especially how
>> we are going to communicate the CQE size to the kernel? I guess just adding
>> IORING_SETUP_CQE256 / IORING_SETUP_CQE512 would be much easier.
> 
> Not Pavel and unfortunately I could not be at that LPC discussion, but
> yeah I don't see why not just adding the necessary SETUP arg for this
> would not be the way to go. As long as they are power-of-2, then all
> it'll impact on both the kernel and liburing side is what size shift to
> use when iterating CQEs.

Thanks, Pavel also wanted power-of-2, although 512 is a bit much for fuse. 
Well, maybe 256 will be sufficient. Going to look into adding that parameter
during the next days.

> 
> Since this obviously means larger CQ rings, one nice side effect is that
> since 6.10 we don't need contig pages to map any of the rings. So should
> work just fine regardless of memory fragmentation, where previously that
> would've been a concern.
> 

Out of interest, what is the change? Up to fuse-io-uring rfc2 I was
vmalloced buffers for fuse that got mmaped - was working fine. Miklos just
wants to avoid that kernel allocates large chunks of memory on behalf of
users.


Thanks,
Bernd