linux-kernel - Re: [PATCH] scsi: Remove some false sharing in Scsi_Host / Scsi

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Message-ID: <20140713114621.GA4316@infradead.org>
Date:	Sun, 13 Jul 2014 04:46:21 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] scsi: Remove some false sharing in Scsi_Host /
 Scsi_Device

On Fri, Jul 11, 2014 at 01:25:10PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> These data structures are accessed by different CPUs and have some
> fields which are mostly read only and others which are frequently
> written. Separate some common ones into separate cache line
> to minimize false sharing while submitting a command.
> 
> This allows scsi_dispatch_cmd to do more work with shared clean
> cache lines.
> 
> - Move the cmd_serial_number to the end before the host data
> - Separate write common fields from read mostly fields in
> the scsi device

Looks useful, but please provide some numbers on how this actually
helps.

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