Re: [PATCH 3/4] btrfs: don't spin in shrink_delalloc if there is nothing to free

[Sergei Trofimovich <slyfox@gentoo.org>]

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On Mon, 09 May 2011 12:00:28 -0400
Josef Bacik <josef@redhat.com> wrote:

> On 05/07/2011 05:29 PM, slyich@gmail.com wrote:
> > Observed as a large delay when --mixed filesystem is filled up.
> > Test example:
> > 1. create tiny --mixed FS:
> >     $ dd if=/dev/zero of=2G.img seek=$((2048 * 1024 * 1024 - 1)) count=1 bs=1
> >     $ mkfs.btrfs --mixed 2G.img
> >     $ mount -oloop 2G.img /mnt/ut/
> > 2. Try to fill it up:
> >     $ dd if=/dev/urandom of=10M.file bs=10240 count=1024
> >     $ seq 1 256 | while read file_no; do echo $file_no; time cp 10M.file ${file_no}.copy; done
> >
> > Up to '200.copy' it goes fast, but when disk fills-up each -ENOSPC
> > message takes 3 seconds to pop-up _every_ ENOSPC (and in usermode linux
> > it's even more: 30-60 seconds!). (Maybe, time depends on kernel's timer resolution).
> >
> > No IO, no CPU load, just rescheduling. Some debugging revealed busy spinning
> > in shrink_delalloc.
> >
> > Signed-off-by: Sergei Trofimovich<slyfox@gentoo.org>
> > ---
> >   fs/btrfs/extent-tree.c |    4 ++++
> >   1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index 9ee6bd5..9f5fdd3 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -3425,6 +3425,10 @@ static int shrink_delalloc(struct btrfs_trans_handle *trans,
> >   	if (reserved == 0)
> >   		return 0;
> >
> > +	/* nothing to shrink - nothing to reclaim */
> > +	if (root->fs_info->delalloc_bytes == 0)
> > +		return 0;
> > +
> >   	max_reclaim = min(reserved, to_reclaim);
> >
> >   	while (loops<  1024) {
> 
> Nice catch, you can add
> 
> Reviewed-by: Josef Bacik <josef@redhat.com>

Will add it (and fix patch enumeration) and resend.

Thanks for the review!

-- 

  Sergei

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