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writeback: per-bdi background threshold 

One thing puzzled me is that in JBOD case, the per-disk writeout
performance is smaller than the corresponding single-disk case even
when they have comparable bdi_thresh. Tracing shows find that in single
disk case, bdi_writeback is always kept high while in JBOD case, it
could drop low from time to time and correspondingly bdi_reclaimable
could sometimes rush high.

The fix is to watch bdi_reclaimable and kick background writeback as
soon as it goes high. This resembles the global background threshold
but in per-bdi manner. The trick is, as long as bdi_reclaimable does
not go high, bdi_writeback naturally won't go low because
bdi_reclaimable+bdi_writeback ~= bdi_thresh.

With less fluctuated writeback pages, JBOD performance is observed to
increase noticeably in various cases.

vmstat:nr_written values before/after patch:

  3.1.0-rc4-wo-underrun+      3.1.0-rc4-bgthresh3+  
------------------------  ------------------------  
               125596480       +25.9%    158179363  JBOD-10HDD-16G/ext4-100dd-1M-24p-16384M-20:10-X
                61790815      +110.4%    130032231  JBOD-10HDD-16G/ext4-10dd-1M-24p-16384M-20:10-X
                58853546        -0.1%     58823828  JBOD-10HDD-16G/ext4-1dd-1M-24p-16384M-20:10-X
               110159811       +24.7%    137355377  JBOD-10HDD-16G/xfs-100dd-1M-24p-16384M-20:10-X
                69544762       +10.8%     77080047  JBOD-10HDD-16G/xfs-10dd-1M-24p-16384M-20:10-X
                50644862        +0.5%     50890006  JBOD-10HDD-16G/xfs-1dd-1M-24p-16384M-20:10-X
                42677090       +28.0%     54643527  JBOD-10HDD-thresh=100M/ext4-100dd-1M-24p-16384M-100M:10-X
                47491324       +13.3%     53785605  JBOD-10HDD-thresh=100M/ext4-10dd-1M-24p-16384M-100M:10-X
                52548986        +0.9%     53001031  JBOD-10HDD-thresh=100M/ext4-1dd-1M-24p-16384M-100M:10-X
                26783091       +36.8%     36650248  JBOD-10HDD-thresh=100M/xfs-100dd-1M-24p-16384M-100M:10-X
                35526347       +14.0%     40492312  JBOD-10HDD-thresh=100M/xfs-10dd-1M-24p-16384M-100M:10-X
                44670723        -1.1%     44177606  JBOD-10HDD-thresh=100M/xfs-1dd-1M-24p-16384M-100M:10-X
               127996037       +22.4%    156719990  JBOD-10HDD-thresh=2G/ext4-100dd-1M-24p-16384M-2048M:10-X
                57518856        +3.8%     59677625  JBOD-10HDD-thresh=2G/ext4-10dd-1M-24p-16384M-2048M:10-X
                51919909       +12.2%     58269894  JBOD-10HDD-thresh=2G/ext4-1dd-1M-24p-16384M-2048M:10-X
                86410514       +79.0%    154660433  JBOD-10HDD-thresh=2G/xfs-100dd-1M-24p-16384M-2048M:10-X
                40132519       +38.6%     55617893  JBOD-10HDD-thresh=2G/xfs-10dd-1M-24p-16384M-2048M:10-X
                48423248        +7.5%     52042927  JBOD-10HDD-thresh=2G/xfs-1dd-1M-24p-16384M-2048M:10-X
               206041046       +44.1%    296846536  JBOD-10HDD-thresh=4G/xfs-100dd-1M-24p-16384M-4096M:10-X
                72312903       -19.4%     58272885  JBOD-10HDD-thresh=4G/xfs-10dd-1M-24p-16384M-4096M:10-X
                50635672        -0.5%     50384787  JBOD-10HDD-thresh=4G/xfs-1dd-1M-24p-16384M-4096M:10-X
                68308534      +115.7%    147324758  JBOD-10HDD-thresh=800M/ext4-100dd-1M-24p-16384M-800M:10-X
                57882933       +14.5%     66269621  JBOD-10HDD-thresh=800M/ext4-10dd-1M-24p-16384M-800M:10-X
                52183472       +12.8%     58855181  JBOD-10HDD-thresh=800M/ext4-1dd-1M-24p-16384M-800M:10-X
                53788956       +94.2%    104460352  JBOD-10HDD-thresh=800M/xfs-100dd-1M-24p-16384M-800M:10-X
                44493342       +35.5%     60298210  JBOD-10HDD-thresh=800M/xfs-10dd-1M-24p-16384M-800M:10-X
                42641209       +18.9%     50681038  JBOD-10HDD-thresh=800M/xfs-1dd-1M-24p-16384M-800M:10-X

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
This commit is contained in:
Wu Fengguang 2010-11-18 14:38:33 -06:00
parent 8927f66c4e
commit b00949aa2d
1 changed files with 12 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
fs/fs-writeback.c
View File

@ -658,14 +658,21 @@ long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages)
return nr_pages - work.nr_pages; return nr_pages - work.nr_pages;
} }
static inline bool over_bground_thresh(void) static bool over_bground_thresh(struct backing_dev_info *bdi)
{ {
unsigned long background_thresh, dirty_thresh; unsigned long background_thresh, dirty_thresh;
global_dirty_limits(&background_thresh, &dirty_thresh); global_dirty_limits(&background_thresh, &dirty_thresh);
return (global_page_state(NR_FILE_DIRTY) + if (global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS) > background_thresh); global_page_state(NR_UNSTABLE_NFS) > background_thresh)
return true;
if (bdi_stat(bdi, BDI_RECLAIMABLE) >
bdi_dirty_limit(bdi, background_thresh))
return true;
return false;
} }
/* /*
@ -727,7 +734,7 @@ static long wb_writeback(struct bdi_writeback *wb,
* For background writeout, stop when we are below the * For background writeout, stop when we are below the
* background dirty threshold * background dirty threshold
*/ */
if (work->for_background && !over_bground_thresh()) if (work->for_background && !over_bground_thresh(wb->bdi))
break; break;
if (work->for_kupdate) { if (work->for_kupdate) {
@ -811,7 +818,7 @@ static unsigned long get_nr_dirty_pages(void)
static long wb_check_background_flush(struct bdi_writeback *wb) static long wb_check_background_flush(struct bdi_writeback *wb)
{ {
if (over_bground_thresh()) { if (over_bground_thresh(wb->bdi)) {
struct wb_writeback_work work = { struct wb_writeback_work work = {
.nr_pages = LONG_MAX, .nr_pages = LONG_MAX,