Aneesh Kumar K.V wrote:
Aneesh Kumar K.V wrote:
Hi All,
I looked at the delalloc and reservation differences that Valerie was
observing.
Below is my understanding. I am not sure whether the below will result
in higher fragmentation that Eric Sandeen is observing. I guess it
should not. Even
though the reservation gets discarded during the clear inode due to
memory pressure
the request for new reservation should get the blocks nearby and not
break extents right ?
any how below is the simple case.
without delalloc the blocks are requested during
prepare_write/write_begin.
That means we enter ext4_new_blocks_old which will call
ext4_try_to_allocate_with_rsv.
Now if there is no reservation for this inode a new one will be
allocated. After
using the blocks this reservation is destroyed during the close via
ext4_release_file
With delalloc the blocks are not requested until we hit
writeback/ext4_da_writepages
That means if we create new file and close them the reservation will
be discarded
during close via ext4_release_file.( Actually there will be nothing to
clear)
Now when we do a sync/or write back. We try to get the block, the
inode will
request for new reservation. This reservation is not discarded untill
we call clear_inode
and that results in the behavior we are seeing.
Free blocks: 1440-8191, 8194-8199, 8202-8207, 8210-8215, 8218-8223,
8226-8231, 8234-8239, 8242-8247, 8250-8255, 8258-8263, 8266-8271,
8274-8279, 8282-8287, 8290-8295, 8298-8303, 8306-8311, 8314-8319,
8322-8327, 8330-8335, 8338-8343, 8346-12799
So now the question is where do we discard the reservation in case of
delalloc.
-
with respect to mballoc we are not seeing this because we are doing
allocation from group prealloc list which is per cpu.
For most the case we have EXT4_MB_HINT_GROUP_ALLOC set in mballoc.
In ext4_mb_group_or_file i already have a FIXME!! regarding this.
currently we have
/* request is so large that we don't care about
* streaming - it overweights any possible seek */
if (ac->ac_o_ex.fe_len >= sbi->s_mb_large_req)
return;
/* FIXME!!
* is this >= considering the above ?
*/
if (ac->ac_o_ex.fe_len >= sbi->s_mb_small_req)
return;
.....
......
/* we're going to use group allocation */
ac->ac_flags |= EXT4_MB_HINT_GROUP_ALLOC;
........
.........
So for small size we have the EXT4_MB_HINT_GROUP_ALLOC set . Now if
i change the the line below FIXME!! to <= , that will force
small size to use inode prealloc and that cause
Free blocks: 1442-1443, 1446-1447, 1450-1451, 1454-1455, 1458-1459,
1462-1463, 1466-1467, 1470-1471, 1474-1475, 1478-1479, 1482-1483,
1486-1487, 1490-1491, 1494-1495, 1498-1499, 1502-1503, 1506-1507,
1510-1511, 1514-1515, 1518-12799
So the problem is generic.
the below patch give ok results with nomballoc.
allocate new block: goal 8192, found 8192/2
allocate new block: goal 8192, found 8194/2
allocate new block: goal 8192, found 8196/2
allocate new block: goal 8192, found 8198/2
allocate new block: goal 8192, found 8200/2
allocate new block: goal 8192, found 8202/2
allocate new block: goal 8192, found 8204/2
allocate new block: goal 8192, found 8206/2
allocate new block: goal 8192, found 8208/2
allocate new block: goal 8192, found 8210/2
allocate new block: goal 8192, found 8212/2
allocate new block: goal 8192, found 8214/2
allocate new block: goal 8192, found 8216/2
allocate new block: goal 8192, found 8218/2
allocate new block: goal 8192, found 8220/2
allocate new block: goal 8192, found 8222/2
allocate new block: goal 8192, found 8224/2
allocate new block: goal 8192, found 8226/2
allocate new block: goal 8192, found 8228/2
allocate new block: goal 8192, found 8230/2
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ac4d032..a3a7205 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1410,7 +1410,14 @@ out:
static int ext4_da_writepages(struct address_space *mapping,
struct writeback_control *wbc)
{
- return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
+ int retval;
+ retval = mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
+ if (!retval) {
+ /* if writepages is successfull discard the reservation */
+ ext4_discard_reservation(mapping->host);
+ }
+
+ return retval;
}
static void ext4_da_invalidatepage(struct page *page, unsigned long offset)
-aneesh
-
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