linux-kernel - Re: [RFC PATCH 0/2] sbitmap: NUMA node spreading

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Date:   Wed, 11 May 2022 10:07:01 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.garry@...wei.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] sbitmap: NUMA node spreading

On Tue, May 10, 2022 at 02:44:50PM +0100, John Garry wrote:
> On 10/05/2022 13:50, Jens Axboe wrote:
> > > fio config:
> > > bs=4096, iodepth=128, numjobs=10, cpus_allowed_policy=split, rw=read,
> > > ioscheduler=none
> > > 
> > > Before:
> > > 7130K
> > > 
> > > After:
> > > 7630K
> > > 
> > > So a +7% IOPS gain.
> 
> Thanks for having a look.
> 
> > What does the comparison run on a non-NUMA non-shared queue look like?
> > Because I bet it'd be slower.
> 
> I could test more to get a solid result for that.
> 
> > 
> > To be honest, I don't like this approach at all. It makes the normal
> > case quite a bit slower by having an extra layer of indirection for the
> > word, that's quite a bit of extra cost.
> 
> Yes, there is the extra load. I would hope that there would be a low cost,
> but I agree that we still want to avoid it. So prob no point in testing this
> more.
> 
> > It doesn't seem like a good
> > approach for the issue, as it pessimizes the normal fast case.
> > 
> > Spreading the memory out does probably make sense, but we need to retain
> > the fast normal case. Making sbitmap support both, selected at init
> > time, would be far more likely to be acceptable imho.
> 
> I wanted to keep the code changes minimal for an initial RFC to test the
> water.
> 
> My original approach did not introduce the extra load for normal path and
> had some init time selection for a normal word map vs numa word map, but the
> code grew and became somewhat unmanageable. I'll revisit it to see how to
> improve that.

I understand this approach just splits shared sbitmap into per-numa-node
part, but what if all IOs are just from CPUs in one same numa node? Doesn't
this way cause tag starvation and waste?


Thanks,
Ming