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ext4: optimize file overwrites

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In case if the file already has underlying blocks/extents allocated
then we don't need to start a journal txn and can directly return
the underlying mapping. Currently ext4_iomap_begin() is used by
both DAX & DIO path. We can check if the write request is an
overwrite & then directly return the mapping information.

This could give a significant perf boost for multi-threaded writes
specially random overwrites.
On PPC64 VM with simulated pmem(DAX) device, ~10x perf improvement
could be seen in random writes (overwrite). Also bcoz this optimizes
away the spinlock contention during jbd2 slab cache allocation
(jbd2_journal_handle). On x86 VM, ~2x perf improvement was observed.

Reported-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/88e795d8a4d5cd22165c7ebe857ba91d68d8813e.1600401668.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Ritesh Harjani 2021-05-03 15:23:17 +02:00 committed by spakkkk
parent 4d8e9a4708
commit 70cc7eeb24
1 changed files with 13 additions and 1 deletions
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14
fs/ext4/inode.c
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@ -3575,6 +3575,18 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
int dio_credits;
handle_t *handle;
int retries = 0;
/*
* We check here if the blocks are already allocated, then we
* don't need to start a journal txn and we can directly return
* the mapping information. This could boost performance
* especially in multi-threaded overwrite requests.
*/
if (offset + length <= i_size_read(inode)) {
ret = ext4_map_blocks(NULL, inode, &map, 0);
if (ret > 0 && (map.m_flags & EXT4_MAP_MAPPED))
goto out;
}
/* Trim mapping request to maximum we can map at once for DIO */
if (map.m_len > DIO_MAX_BLOCKS)
@ -3601,7 +3613,7 @@ retry:
goto retry;
return ret;
}
out:
/*
* If we added blocks beyond i_size, we need to make sure they
* will get truncated if we crash before updating i_size in