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btrfs: don't spin in shrink_delalloc if there is nothing to free

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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>
Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
This commit is contained in:
Sergei Trofimovich 2011-05-20 20:20:30 +00:00 committed by Chris Mason
parent 0f3b708c11
commit c4f675cd40
1 changed files with 4 additions and 0 deletions
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4
fs/btrfs/extent-tree.c
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@ -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) {