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Message-ID: <27aae921-d016-0416-17ff-a591cd2ae12d@huaweicloud.com>
Date: Mon, 28 Nov 2022 11:09:28 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: John Garry <john.g.garry@...cle.com>, kashyap.desai@...adcom.com,
sumit.saxena@...adcom.com, shivasharan.srikanteshwara@...adcom.com,
jejb@...ux.ibm.com, martin.petersen@...cle.com,
megaraidlinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
linux-kernel@...r.kernel.org,
linux-block <linux-block@...r.kernel.org>,
"zhangyi (F)" <yi.zhang@...wei.com>,
"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: Why is MEGASAS_SAS_QD set to 256?
Hi,
在 2022/11/27 17:42, Ming Lei 写道:
> On Sat, Nov 26, 2022 at 02:08:02PM +0800, Yu Kuai wrote:
>> Hi, Ming
>>
>> 在 2022/11/26 10:18, Ming Lei 写道:
>>>
>>> If you want aggressive merge on sequential IO workload, the queue depth need
>>> to be a bit less, then more requests can be staggered into scheduler queue,
>>> and merge chance is increased.
>>
>> But if nr_requests >= queue_depth, it seems to me elevator will have no
>> effect, no request can be merged or sorted by scheduler, right?
>
> Yeah.
>
> If nr_requests <= queue_depth, every request can be queued to
> driver/device, so requests won't be merged by scheduler.
>
> But plug merge still works if IOs are submitted as batch.
Yes, io can still be merged by plug. I just find it a little werid to
set default elevator as deadline, and default queue_depth to 256. Which
means deadline here is useless.
>
>>>
>>> If you want good perf on random IO perf, the queue depth needs to
>>> be deep enough to have enough parallelism for saturating SSD internal.
>>>
>>> But we don't recognize sequential/random IO pattern, and usually fixed
>>> queue depth is used.
>>
>> Is it possible to use none elevator and set large queue_depth if nvme is
>> used in this case?
>
> Yeah, if the storage is SSD, usually none with bigger queue_depth should
> help, and usually 256 should be enough to saturate one single SSD for
> one well implemented driver.
Yes, I'm testing with multiple SSDs / NVMEs, and I can't get optimal
performance by default.
Thanks,
Kuai
>
>
> Thanks
> Ming
>
> .
>
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