Rolf Fokkens
2014-05-18 08:23:30 UTC
Sometimes bcache_writebac task consumes 100% CPU during normal operation, but after a while (like a minute) this drops to normal.
It can be triggered in a reproducable fashion on my system by:
echo 0 > /sys/block/bcache0/bcache/writeback_percent
After this bcache_writebac starts to burn CPU cycles, and it never seems to stop doing so. changing the writeback_percent value back to 25 doesn't change anything.
Apart from a busy CPU there appear to be no problem (e.g. no "task bcache_writebac:286 blocked for more than 120 seconds" messages).
I filed this issue also on: https://bugzilla.kernel.org/show_bug.cgi?id=76391
[***@home07 ~]$ bcache-status -s
--- bcache ---
UUID 4892b3d5-fd49-4d71-adb6-d7b0a799e913
Block Size 512 B
Bucket Size 512.00 KiB
Congested? False
Read Congestion 0.0ms
Write Congestion 0.0ms
Total Cache Size 30 GiB
Total Cache Used 2 GiB (6%)
Total Cache Unused 28 GiB (94%)
Evictable Cache 30 GiB (100%)
Replacement Policy [lru] fifo random
Cache Mode writethrough [writeback] writearound none
Total Hits 31727 (99%)
Total Misses 76
Total Bypass Hits 3569 (100%)
Total Bypass Misses 0
Total Bypassed 27.5 MiB
--- Backing Device ---
Device File /dev/md2 (9:2)
bcache Device File /dev/bcache0 (252:0)
Size 140 GiB
Cache Mode writethrough [writeback] writearound none
Readahead 0
Sequential Cutoff 0 B
Merge sequential? False
State dirty
Writeback? True
Dirty Data 6.1 MiB
Total Hits 31727 (99%)
Total Misses 76
Total Bypass Hits 3569 (100%)
Total Bypass Misses 0
Total Bypassed 27.5 MiB
--- Cache Device ---
Device File /dev/sdd1 (8:49)
Size 30 GiB
Block Size 512 B
Bucket Size 512.00 KiB
Replacement Policy [lru] fifo random
Discard? True
I/O Errors 0
Metadata Written 906.00 KiB
Data Written 52.3 MiB
Buckets 61440
Cache Used 2 GiB (6%)
Cache Unused 28 GiB (94%)
[***@home07 ~]$
It can be triggered in a reproducable fashion on my system by:
echo 0 > /sys/block/bcache0/bcache/writeback_percent
After this bcache_writebac starts to burn CPU cycles, and it never seems to stop doing so. changing the writeback_percent value back to 25 doesn't change anything.
Apart from a busy CPU there appear to be no problem (e.g. no "task bcache_writebac:286 blocked for more than 120 seconds" messages).
I filed this issue also on: https://bugzilla.kernel.org/show_bug.cgi?id=76391
[***@home07 ~]$ bcache-status -s
--- bcache ---
UUID 4892b3d5-fd49-4d71-adb6-d7b0a799e913
Block Size 512 B
Bucket Size 512.00 KiB
Congested? False
Read Congestion 0.0ms
Write Congestion 0.0ms
Total Cache Size 30 GiB
Total Cache Used 2 GiB (6%)
Total Cache Unused 28 GiB (94%)
Evictable Cache 30 GiB (100%)
Replacement Policy [lru] fifo random
Cache Mode writethrough [writeback] writearound none
Total Hits 31727 (99%)
Total Misses 76
Total Bypass Hits 3569 (100%)
Total Bypass Misses 0
Total Bypassed 27.5 MiB
--- Backing Device ---
Device File /dev/md2 (9:2)
bcache Device File /dev/bcache0 (252:0)
Size 140 GiB
Cache Mode writethrough [writeback] writearound none
Readahead 0
Sequential Cutoff 0 B
Merge sequential? False
State dirty
Writeback? True
Dirty Data 6.1 MiB
Total Hits 31727 (99%)
Total Misses 76
Total Bypass Hits 3569 (100%)
Total Bypass Misses 0
Total Bypassed 27.5 MiB
--- Cache Device ---
Device File /dev/sdd1 (8:49)
Size 30 GiB
Block Size 512 B
Bucket Size 512.00 KiB
Replacement Policy [lru] fifo random
Discard? True
I/O Errors 0
Metadata Written 906.00 KiB
Data Written 52.3 MiB
Buckets 61440
Cache Used 2 GiB (6%)
Cache Unused 28 GiB (94%)
[***@home07 ~]$