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Subject: Re: [PATCH -next RFC 0/6] improve large random io for HDD
To:     Jens Axboe <axboe@kernel.dk>, <andriy.shevchenko@linux.intel.com>,
        <john.garry@huawei.com>, <ming.lei@redhat.com>
CC:     <linux-block@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <yi.zhang@huawei.com>
References: <20220329094048.2107094-1-yukuai3@huawei.com>
 <a3e78af2-b0e3-9a97-5bbd-4bdbc5c5a58d@kernel.dk>
From:   "yukuai (C)" <yukuai3@huawei.com>
Message-ID: <7dec80e4-b96c-f00e-10c0-8510efa4b572@huawei.com>
Date:   Wed, 30 Mar 2022 10:05:53 +0800
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在 2022/03/29 20:53, Jens Axboe 写道:
> On 3/29/22 3:40 AM, Yu Kuai wrote:
>> There is a defect for blk-mq compare to blk-sq, specifically split io
>> will end up discontinuous if the device is under high io pressure, while
>> split io will still be continuous in sq, this is because:
>>
>> 1) split bio is issued one by one, if one bio can't get tag, it will go
>> to wail. - patch 2
>> 2) each time 8(or wake batch) requests is done, 8 waiters will be woken up.
>> Thus if a thread is woken up, it will unlikey to get multiple tags.
>> - patch 3,4
>> 3) new io can preempt tag even if there are lots of threads waiting for
>> tags. - patch 5
>>
>> Test environment:
>> x86 vm, nr_requests is set to 64, queue_depth is set to 32 and
>> max_sectors_kb is set to 128.
>>
>> I haven't tested this patchset on physical machine yet, I'll try later
>> if anyone thinks this approch is meaningful.
> 
> A real machine test would definitely be a requirement. What real world
> uses cases is this solving? These days most devices have plenty of tags,
> and I would not really expect tag starvation to be much of a concern.
> 
> However, I do think there's merrit in fixing the unfairness we have
> here. But not at the cost of all of this. Why not just simply enforce
> more strict ordering of tag allocations? If someone is waiting, you get
> to wait too.
> 
> And I don't see much utility at all in tracking how many splits (and
> hence tags) would be required. Is this really a common issue, tons of
> splits and needing many tags? Why not just enforce the strict ordering
> as mentioned above, not allowing new allocators to get a tag if others
> are waiting, but perhaps allow someone submitting a string of splits to
> indeed keep allocating.
> 
> Yes, it'll be less efficient to still wake one-by-one, but honestly do
> we really care about that? If you're stalled on waiting for other IO to
> finish and release a tag, that isn't very efficient to begin with and
> doesn't seem like a case worth optimizing for me.
> 

Hi,

Thanks for your adivce, I'll do more work based on your suggestions.

Kuai