linux-kernel - Re: [PATCH RESEND] fs: Move bh_cachep to the __read

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Date:	Thu, 7 Jun 2012 17:07:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vlad Zolotarov <vlad@...lemp.com>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Shai@...lemp.com
Subject: Re: [PATCH RESEND] fs: Move bh_cachep to the __read_mostly section

On Mon, 28 May 2012 14:58:42 +0300
Vlad Zolotarov <vlad@...lemp.com> wrote:

> From: Shai Fultheim <shai@...lemp.com>
> 
> bh_cachep is only written to once on initialization, so move it to the
> __read_mostly section.
> 
> Signed-off-by: Shai Fultheim <shai@...lemp.com>
> Signed-off-by: Vlad Zolotarov <vlad@...lemp.com>
> ---
>  fs/buffer.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index ad5938c..838a9cf 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3152,7 +3152,7 @@ SYSCALL_DEFINE2(bdflush, int, func, long, data)
>  /*
>   * Buffer-head allocation
>   */
> -static struct kmem_cache *bh_cachep;
> +static struct kmem_cache *bh_cachep __read_mostly;
>  

hm, I thought I replied to this earlier, but I can't find that email.

Yes, bh_cachep is read-mostly.  In fact it's write-once.  But the same
is true of all kmem_cache*'s.  I don't see any particular reason for
singling out bh_cachep.


Alas, I don't see a smart way of addressing this.  It's either a
patchset which adds __read_mostly to all kmem_cache*'s, or a patchset
which converts all the definitions to use some nasty macro which
inserts the __read_mostly.



And I still have theoretical concerns with __read_mostly.  As we
further sort the storage into read-mostly and write-often sections, the
density of writes in the write-mostly section increases.  IOW, removing
the read-mostly padding *increase* cross-CPU traffic in the write-often
scction.  IOW2, leaving the read-mostly stuff where it is provides
beneficial padding to the write-often fields.  I don't think it has
been shown that there will be net gains.
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