Re: Large increase in context switch rate

[Peter Zijlstra]

On Fri, 2008-07-25 at 12:31 +0100, Alex Nixon wrote:
> >> I've bisected down to commit ba52de123d454b57369f291348266d86f4b35070 -
> >> [PATCH] inode-diet.  Before that kernbench consistently reports about
> >> 35k context switches (total), and after that commit about 53k.  The
> >> benchmarks are being run on a tmpfs.  I've verified the results on a
> >> different machine, albeit with an almost identical setup (the same
> >> kernels and debian distro, kernbench version, and benchmarking a build
> >> of the same source).
> >>
> >> Seems to be a mystery why that patch is (seemingly) the culprit...
> 
> The relevant changeset had caused the blocksize to default to 1024 (as opposed
> to 4096) - as a result there was a large increase in the time spent waiting on
> pipes.
> 
> Instead of re-adding the line taken out of fs/pipe.c by Theodore I opted instead
> to change the default block size for pseudo-filesystems to PAGE_SIZE, to try
> avoid making pipe.c inconsistent with Theodore's new approach.
> 
> The performance penalty from these extra context switches is fairly small, but 
> is magnified when virtualization is involved, hence the desire to keep it lower
> if possible.
> 
> 
> From 4b568a72fc42b52279507eb4d1339e0637ae719a Mon Sep 17 00:00:00 2001
> From: Alex Nixon <t_alexn@alexn-desktop.(none)>
> Date: Fri, 25 Jul 2008 11:26:44 +0100
> Subject: [PATCH] VFS: increase pseudo-filesystem block size to PAGE_SIZE.
> 
> Changeset ba52de123d454b57369f291348266d86f4b35070 caused the block size used
> by pseudo-filesystems to decrease from PAGE_SIZE to 1024 leading to a doubling
> of the number of context switches during a kernbench run.

Cool - makes sense. I'd ack it, but I know less than nothing about this
code, so I won't...

Still, good hunting on your part!

> Signed-off-by: Alex Nixon <Alex.Nixon@xxxxxxxxxx>
> ---
>  fs/libfs.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index baeb71e..1add676 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -216,8 +216,8 @@ int get_sb_pseudo(struct file_system_type *fs_type, char *name,
>  
>  	s->s_flags = MS_NOUSER;
>  	s->s_maxbytes = ~0ULL;
> -	s->s_blocksize = 1024;
> -	s->s_blocksize_bits = 10;
> +	s->s_blocksize = PAGE_SIZE;
> +	s->s_blocksize_bits = PAGE_SHIFT;
>  	s->s_magic = magic;
>  	s->s_op = ops ? ops : &simple_super_operations;
>  	s->s_time_gran = 1;

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