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

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Date:   Tue, 10 May 2022 16:03:21 +0100
From:   John Garry <john.garry@...wei.com>
To:     Jens Axboe <axboe@...nel.dk>, <linux-block@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>
Subject: Re: [RFC PATCH 0/2] sbitmap: NUMA node spreading

On 10/05/2022 15:34, Jens Axboe wrote:
>> 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.
> I don't think that's low cost at all. It's the very hot path, and you're
> now not only doing an extra load, it's a dependent load - you need to
> load both to make any progress. On top of that, it's not like it's two
> loads from the same cacheline or even page. The most important thing for
> performance these days is having good cache utilization, the patch as it
> stands very much makes that a lot worse.

Understood. Essentially patch #1/2 points in the wrong direction.

I have to admit that I was a bit blinkered by seeing how much I could 
improve the NUMA case.

> 
> Besides, for any kind of performance work like that, it's customary to
> showcase both the situation that is supposedly fixed or improved with
> the change, but also to test that it didn't regress the existing
> common/fast case.

Right, I should have done that.

> 
>>> 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.
> Probably just needs some clean refactoring first, so that the actual
> change can be pretty small.

I think that it may be just a case of separating out the handling of 
evaluating the sbitmap_word ptr as that is that common struct which we 
deal with.

Thanks,
John