On May 24, 2011, at 08:16, Martin Tegtmeier wrote:
> currently dropping the file system cache ("echo 1 >
> /proc/sys/vm/drop_caches") doesn't leave any trace. However dropping
> the fs cache can severely impact system behaviour and application
> response times. Therefore I suggest to write a syslog entry if the
> entire inode page cache is scrapped.
> Since it is not an easy task to calculate the size of the droppable
> filesystem cache I also suggest to add the number of dropped pages to
> the syslog entry. This can be accomplished by saving the return value
> of invalidate_mapping_pages().
>
> The number of dropped pages is an important measure for capacity
> planning. For the deployment of new SAP application instances we
> would like to know the amount of memory that was freed from
> fs caches.
I'm hugely in favour of this, because we don't need syslog to be an
auditing mechanism for userspace.
However, if something like this goes into the kernel it would probably
make a lot more sense to print out the values on a per-filesystem
basis, if any pages are dropped from a particular filesystem, so one
can see which filesystem was using the most cache.
> drop_caches.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
>
> commit b2219e84647bdf64fd6e7f9c5260c1e6bed24d58
> Author: Martin Tegtmeier <martin.tegtmeier@xxxxxxxxx>
> Date: Tue May 24 15:24:20 2011 +0200
>
> drop_caches: add syslog entry
>
> Dropping the entire file system cache (inode cache) can severely
> influence system behaviour
> yet currently dropping the file system cache is NOT traceable.
> This patch adds an entry to /var/log/messages with a time stamp
> and the number of dropped pages.
>
>
> Signed-off-by: Martin Tegtmeier <martin.tegtmeier@xxxxxxxxx>
>
> diff --git a/fs/drop_caches.c b/fs/drop_caches.c
> index 98b77c8..f2e4dc4 100644
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -12,6 +12,7 @@
>
> /* A global variable is a bit ugly, but it keeps the code simple */
> int sysctl_drop_caches;
> +unsigned long pages_dropped;
>
> static void drop_pagecache_sb(struct super_block *sb, void *unused)
> {
> @@ -28,7 +29,7 @@ static void drop_pagecache_sb(struct super_block
> *sb, void *unused)
> __iget(inode);
> spin_unlock(&inode->i_lock);
> spin_unlock(&inode_sb_list_lock);
> - invalidate_mapping_pages(inode->i_mapping, 0, -1);
> + pages_dropped += invalidate_mapping_pages(inode->i_mapping, 0, -1);
> iput(toput_inode);
> toput_inode = inode;
> spin_lock(&inode_sb_list_lock);
> @@ -55,8 +56,12 @@ int drop_caches_sysctl_handler(ctl_table *table, int write,
> if (ret)
> return ret;
> if (write) {
> - if (sysctl_drop_caches & 1)
> + if (sysctl_drop_caches & 1) {
> + pages_dropped = 0;
> iterate_supers(drop_pagecache_sb, NULL);
> + printk(KERN_INFO "drop_caches: %lu pages dropped from inode cache\n",
> + pages_dropped);
> + }
> if (sysctl_drop_caches & 2)
> drop_slab();
> }
> --
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Cheers, Andreas
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