Re: Large increase in context switch rate
From: Jeremy Fitzhardinge
Date: Fri Jul 25 2008 - 10:54:22 EST
Alex Nixon wrote:
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.
Good work!
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.
Probably worth explicitly noting the effect on pipe buffer size.
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;
J
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