linux-kernel - Re: [patch] block: make struct block_device cacheline

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Date:	Wed, 19 Sep 2012 11:50:59 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
cc:	jaxboe@...ionio.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] block: make struct block_device cacheline_aligned



On Wed, 19 Sep 2012, Jeff Moyer wrote:

> Mikulas Patocka <mpatocka@...hat.com> writes:
> 
> > On Wed, 19 Sep 2012, Jeff Moyer wrote:
> >
> >> Jeff Moyer <jmoyer@...hat.com> writes:
> >> 
> >> > Hi,
> >> >
> >> > When testing against a pcie ssd or a ramdisk, making the block device
> >> > structure cacheline_aligned provided a significant increase in
> >> > performance:
> >> 
> >> Self-NACK on this one.  This results in a ton of warnings:
> >> 
> >> include/linux/fs.h:727: warning: ???__section__??? attribute does not
> >> apply to types
> >> In file included from include/linux/debugfs.h:18,
> >>                  from kernel/trace/trace_probe.h:28,
> >>                  from kernel/trace/trace_kprobe.c:23:
> >> include/linux/fs.h:727: warning: ???__section__??? attribute does not
> >> apply to types
> >> 
> >> And that leaves me with the task of figuring out if/why this actually
> >> helps.
> >> 
> >> Cheers,
> >> Jeff
> >
> > Hi
> >
> > Use ____cacheline_aligned instead of __cacheline_aligned
> 
> struct block_device is allocated as part of the bdev_inode:
> 
> struct bdev_inode {
>         struct block_device bdev;
>         struct inode vfs_inode;
> };
> 
> The bdev_inode is allocated from the bdev_cachep, which uses
> SLAB_HWCACHE_ALIGN.  So, in theory, this should already be aligned.
> 
> -Jeff

The purpose here is to align vfs_inode. If you add alignment to bdev, 
vfs_inode would be aligned (because bdev size would be aligned to 
cacheline boundary). Or you can add the alignment to vfs_inode, it would 
have the same effect.

Mikulas
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