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block_write_full_page: switch synchronous writes to use WRITE_SYNC_PLUG

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Now that we have a distinction between WRITE_SYNC and WRITE_SYNC_PLUG,
use WRITE_SYNC_PLUG in __block_write_full_page() to avoid unplugging
the block device I/O queue between each page that gets flushed out.

Otherwise, when we run sync() or fsync() and we need to write out a
large number of pages, the block device queue will get unplugged
between for every page that is flushed out, which will be a pretty
serious performance regression caused by commit a64c8610.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
This commit is contained in:
Theodore Ts'o 2009-04-07 18:12:43 -04:00
parent 577c9c456f
commit 6e34eeddf7
1 changed files with 12 additions and 1 deletions
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13
fs/buffer.c
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@ -1596,6 +1596,16 @@ EXPORT_SYMBOL(unmap_underlying_metadata);
* locked buffer. This only can happen if someone has written the buffer
* directly, with submit_bh(). At the address_space level PageWriteback
* prevents this contention from occurring.
*
* If block_write_full_page() is called with wbc->sync_mode ==
* WB_SYNC_ALL, the writes are posted using WRITE_SYNC_PLUG; this
* causes the writes to be flagged as synchronous writes, but the
* block device queue will NOT be unplugged, since usually many pages
* will be pushed to the out before the higher-level caller actually
* waits for the writes to be completed. The various wait functions,
* such as wait_on_writeback_range() will ultimately call sync_page()
* which will ultimately call blk_run_backing_dev(), which will end up
* unplugging the device queue.
*/
static int __block_write_full_page(struct inode *inode, struct page *page,
get_block_t *get_block, struct writeback_control *wbc)
@ -1606,7 +1616,8 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
struct buffer_head *bh, *head;
const unsigned blocksize = 1 << inode->i_blkbits;
int nr_underway = 0;
int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);
int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
WRITE_SYNC_PLUG : WRITE);
BUG_ON(!PageLocked(page));