linux-kernel - Re: Why is MEGASAS_SAS

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Message-ID: <aef69f3b-a8db-f34c-4a52-49ba9020f6cf@huaweicloud.com>
Date:   Sat, 26 Nov 2022 14:08:02 +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, 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?
> 
> 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?

Thansk,
Kuai
> 
> Thanks,
> Ming
> 
> .
>