From: James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>
To: Bart Van Assche <bvanassche-HInyCGIudOg@public.gmane.org>
Cc: Sagi Grimberg <sagig-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
michaelc-hcNo3dDEHLuVc3sceRu5cw@public.gmane.org,
martin.petersen-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org,
nab-IzHhD5pYlfBP7FQvKIMDCQ@public.gmane.org,
roland-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org,
linux-scsi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
target-devel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: [PATCH v2 1/3] scsi_cmnd: Introduce scsi_transfer_length helper
Date: Thu, 26 Jun 2014 10:55:51 -0400 [thread overview]
Message-ID: <1403794551.3572.6.camel@dabdike> (raw)
In-Reply-To: <53AC3402.2080302-HInyCGIudOg@public.gmane.org>
On Thu, 2014-06-26 at 16:53 +0200, Bart Van Assche wrote:
> On 06/11/14 11:09, Sagi Grimberg wrote:
> > + return xfer_len + (xfer_len >> ilog2(sector_size)) * 8;
>
> Sorry that I just noticed this now, but why is a shift-right and ilog2()
> used in the above expression instead of just dividing the transfer
> length by the sector size ?
It's a performance thing. Division is really slow on most CPUs.
However, we know the divisor is a power of two so we re-express the
division as a shift, which the processor can do really fast.
James
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next prev parent reply other threads:[~2014-06-26 14:55 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-11 9:09 [PATCH v2 0/3] Include protection information in transport header Sagi Grimberg
2014-06-11 9:09 ` [PATCH v2 1/3] scsi_cmnd: Introduce scsi_transfer_length helper Sagi Grimberg
2014-06-11 23:39 ` Martin K. Petersen
2014-06-23 21:24 ` Mike Christie
[not found] ` <53A89B0F.4040300-hcNo3dDEHLuVc3sceRu5cw@public.gmane.org>
2014-06-24 1:58 ` Martin K. Petersen
2014-06-25 1:17 ` Vladislav Bolkhovitin
2014-07-27 8:45 ` Boaz Harrosh
[not found] ` <1402477799-24610-2-git-send-email-sagig-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
2014-06-24 6:54 ` Mike Christie
2014-06-24 12:53 ` Martin K. Petersen
2014-06-24 14:03 ` Christoph Hellwig
[not found] ` <yq1mwd2h3ju.fsf-+q57XtR/GgMb6DWv4sQWN6xOck334EZe@public.gmane.org>
2014-06-24 13:08 ` Sagi Grimberg
2014-06-24 14:55 ` Christoph Hellwig
2014-06-24 15:29 ` sagi grimberg
2014-06-24 16:08 ` Michael Christie
2014-06-24 16:27 ` Christoph Hellwig
2014-06-24 16:27 ` Sagi Grimberg
2014-06-24 16:30 ` Christoph Hellwig
[not found] ` <20140624163040.GA11499-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2014-06-24 17:00 ` Mike Christie
2014-06-24 17:04 ` Martin K. Petersen
2014-06-24 17:08 ` Mike Christie
[not found] ` <53A9B0A0.6000103-hcNo3dDEHLuVc3sceRu5cw@public.gmane.org>
2014-06-25 3:32 ` Mike Christie
2014-06-25 8:48 ` Sagi Grimberg
2014-06-25 9:17 ` Christoph Hellwig
2014-06-25 10:32 ` Sagi Grimberg
[not found] ` <53AAA547.40300-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>
2014-06-25 11:35 ` Christoph Hellwig
[not found] ` <20140625113536.GA30312-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2014-06-25 15:59 ` Michael Christie
2014-07-27 9:11 ` Boaz Harrosh
[not found] ` <53D4C22F.8050904-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2014-07-27 13:52 ` Christoph Hellwig
2014-08-06 12:15 ` Sagi Grimberg
2014-06-25 9:14 ` Christoph Hellwig
2014-06-25 11:29 ` Martin K. Petersen
2014-06-24 16:31 ` Martin K. Petersen
[not found] ` <yq1fviugtgq.fsf-+q57XtR/GgMb6DWv4sQWN6xOck334EZe@public.gmane.org>
2014-06-24 17:05 ` Mike Christie
2014-06-24 13:01 ` sagi grimberg
2014-06-26 14:53 ` Bart Van Assche
[not found] ` <53AC3402.2080302-HInyCGIudOg@public.gmane.org>
2014-06-26 14:55 ` James Bottomley [this message]
2014-06-26 15:41 ` Atchley, Scott
2014-06-26 16:38 ` James Bottomley
[not found] ` <fbbc6688-5a52-4437-93b1-71e8ff84c36c-2ueSQiBKiTY7tOexoI0I+QC/G2K4zDHf@public.gmane.org>
2014-06-26 21:17 ` Atchley, Scott
2014-07-13 11:37 ` Christoph Hellwig
2014-07-13 11:40 ` Martin K. Petersen
2014-07-25 20:00 ` [PATCH] [SCSI] Make scsi_transfer_length take a scsi_data_buffer argument Martin K. Petersen
2014-07-25 21:19 ` Christoph Hellwig
2014-07-29 13:26 ` Christoph Hellwig
2014-08-06 12:12 ` Sagi Grimberg
2014-08-06 13:09 ` Sagi Grimberg
2014-08-06 15:49 ` Martin K. Petersen
2014-06-11 9:09 ` [PATCH v2 2/3] libiscsi, iser: Adjust data_length to include protection information Sagi Grimberg
2014-06-23 20:59 ` Christoph Hellwig
[not found] ` <20140623205948.GA15165-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2014-06-24 6:31 ` Mike Christie
[not found] ` <1402477799-24610-3-git-send-email-sagig-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
2014-07-27 10:08 ` Boaz Harrosh
[not found] ` <53D4CFAB.3040804-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2014-08-06 12:43 ` Sagi Grimberg
2014-08-06 13:25 ` Boaz Harrosh
2014-08-13 13:09 ` Sagi Grimberg
[not found] ` <53EB639C.3080307-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>
2014-08-14 7:17 ` Boaz Harrosh
[not found] ` <53E22300.3090907-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>
2014-08-06 15:54 ` Martin K. Petersen
2014-06-11 9:09 ` [PATCH v2 3/3] TARGET/sbc,loopback: Adjust command data length in case pi exists on the wire Sagi Grimberg
2014-06-11 21:36 ` [PATCH v2 0/3] Include protection information in transport header Nicholas A. Bellinger
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