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Date:   Wed, 11 May 2022 10:57:45 +0100
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Subject: Re: [RFC PATCH 0/2] sbitmap: NUMA node spreading
To:     Ming Lei <ming.lei@redhat.com>
CC:     Jens Axboe <axboe@kernel.dk>, <linux-block@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-scsi@vger.kernel.org>
References: <1652181274-136198-1-git-send-email-john.garry@huawei.com>
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 <9ede7211-ae58-5cd4-4cf6-74c1f508f1a6@huawei.com> <YnsaRXzKR5FKjC66@T590>
From:   John Garry <john.garry@huawei.com>
In-Reply-To: <YnsaRXzKR5FKjC66@T590>
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On 11/05/2022 03:07, Ming Lei wrote:

Hi Ming,

>>> 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?
> 

We would not do this. If we can't find a free bit in one node then we 
need to check the others before giving up. This is some of the added 
complexity which I hinted at. And things like batch get or RR support 
become more complex.

Alternatively we could have the double pointer for numa spreading only, 
which would make things simpler. I need to check which is overall 
better. Adding the complexity for dealing with numa node sub-arrays may 
affect performance also.

Thanks,
John