linux-kernel - Re: [PATCH] tcmu: Recalculate the tcmu

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Date:   Tue, 02 May 2017 20:40:15 -0700
From:   "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:     Mike Christie <mchristi@...hat.com>
Cc:     lixiubo@...s.chinamobile.com, agrover@...hat.com,
        namei.unix@...il.com, sheng@...ker.org, linux-scsi@...r.kernel.org,
        target-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcmu: Recalculate the tcmu_cmd size to save cmd area
 memories

On Tue, 2017-05-02 at 21:06 -0500, Mike Christie wrote:
> On 05/02/2017 02:54 AM, lixiubo@...s.chinamobile.com wrote:
> > From: Xiubo Li <lixiubo@...s.chinamobile.com>
> > 
> > For the "struct tcmu_cmd_entry" in cmd area, the minimum size
> > will be sizeof(struct tcmu_cmd_entry) == 112 Bytes. And it could
> > fill about (sizeof(struct rsp) - sizeof(struct req)) /
> > sizeof(struct iovec) == 68 / 16 ~= 4 data regions(iov[4]) by
> > default.
> > 
> > For most tcmu_cmds, the data block indexes allocated from the
> > data area will be continuous. And for the continuous blocks they
> > will be merged into the same region using only one iovec. For
> > the current code, it will always allocates the same number of
> > iovecs with blocks for each tcmu_cmd, and it will wastes much
> > memories.
> > 
> > For example, when the block size is 4K and the DATA_OUT buffer
> > size is 64K, and the regions needed is less than 5(on my
> > environment is almost 99.7%). The current code will allocate
> > about 16 iovecs, and there will be (16 - 4) * sizeof(struct
> > iovec) = 192 Bytes cmd area memories wasted.
> > 
> > Here adds two helpers to calculate the base size and full size
> > of the tcmu_cmd. And will recalculate them again when it make sure
> > how many iovs is needed before insert it to cmd area.
> > 
> > Signed-off-by: Xiubo Li <lixiubo@...s.chinamobile.com>
> 
> Looks ok to me. Thanks.
> 
> Acked-by: Mike Christie <mchristi@...hat.com>

Applied.  Thanks Xiubo + MNC.