linux-kernel - Re: [RFC PATCH] nvme-pci: Move the sg table allocation/free into init/exit

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Message-ID: <20200629013143.GA1778220@dhcp-10-100-145-180.wdl.wdc.com>
Date:   Sun, 28 Jun 2020 18:31:43 -0700
From:   Keith Busch <kbusch@...nel.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     axboe@...com, hch@....de, sagi@...mberg.me, baolin.wang7@...il.com,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] nvme-pci: Move the sg table allocation/free into
 init/exit_request

On Sun, Jun 28, 2020 at 06:34:46PM +0800, Baolin Wang wrote:
> Move the sg table allocation and free into the init_request() and
> exit_request(), instead of allocating sg table when queuing requests,
> which can benefit the IO performance.

If you want to pre-allocate something per-request, you can add the size
to the tagset's cmd_size.

But this is adding almost 4k per request. Considering how many requests
we try to allocate, that's a bit too large to count on being available
or sequestor for this driver.