On Mon, 3 Jul 2017, tang.junhui@xxxxxxxxxx wrote: > Hello Eric, Coly > > > So usually it takes 1.4s, as much as 7s on our systems. Average frequency > > is almost an hour. Can GC just be triggered more frequently? Say, once > > every 5min? Is that configurable? > > GC is triggered by writing c->sb.bucket_size * c->nbuckets / 16 cache > data, or triggered by invalidating when there is not enough free > buckets. So GC time is not configurable, and I also do not think it is > usable by triggering GC more frequently, because as debug the code, in > my test bcache version, I find most time used in btree_gc_mark_node() > (in the latest bcache version it maybe btree_gc_mark_node() and > btree_gc_count_keys()), They are all memory operations, can we optimize > it? Perhaps. What is the total lock time that prevents IO? Can you note those below? -- Eric Wheeler > > test result： > > Jul 3 13:38:27 ceph192-9-9-153 kernel: [10541.541876] gc trigged by sectors_to_gc > Jul 3 13:38:27 ceph192-9-9-153 kernel: [10541.544372] bch_gc_thread works > Jul 3 13:38:27 ceph192-9-9-153 kernel: [10541.569479] gc begin btree_root > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.883089] add cl ffff880458627b18 to wait:ffff8804583237f0 > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.884319] journal_write_root with journal_write_unlocked > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.885522] in journal_write_unlocked_root > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.886770] bch_btree_set_root_gc closure_sync wait cl: > ffff880458627b18 > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.886852] journal_write_done_root wake up wait: ffff8804583237f0 > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.889190] bch_btree_set_root_gc closure_sync wait cl: > ffff880458627b18 end > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.892783] gc btree_root over, begin bch_btree_gc_finish > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.895303] mark_node: 7203 times, 7074 ms; btree_alloc: 0 times, 0 > ms; coalse: 7203 times, 0 ms; write1: 7202 times, 80 ms; write2: 1 times, 0 ms; sort_node: 1 times, 1 ms > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.926504] gc bch_btree_gc_finish over, wake_up_allocators > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.928720] bch_moving_gc over, aftre gc, available: 4399506, > nbuckets:6104768 > Jul 3 13:38:34 ceph192-9-9-153 kernel: [10548.930144] bch_gc_thread works over > I am very anxious about this question, any comment would be valuable. > > Regards > Tang Junhui > > > > > > 发件人: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> > 收件人: Coly Li <i@xxxxxxx>, > 抄送: tang.junhui@xxxxxxxxxx, kent.overstreet@xxxxxxxxx, linux-bcache@xxxxxxxxxxxxxxx > 日期: 2017/06/28 08:00 > 主题: Re: Question about gc time > > _________________________________________________________________________________________________________________ > > > > On Wed, 28 Jun 2017, Coly Li wrote: > > > On 2017/6/27 下午8:04, tang.junhui@xxxxxxxxxx wrote: > > > Hello Eric, Coly, > > > > > > I use a 1400G SSD device a bcache cache device, > > > and attach with 10 back-end devices, > > > and run random small write IOs, > > > when gc works, It takes about 15 seconds, > > > and the up layer application IOs was suspended at this time, > > > How could we bear such a long time IO stopping? > > > Is there any way we can avoid this problem? > > > > > > I am very anxious about this question, any comment would be valuable. > > > > I encounter same situation too. > > Hmm, I assume there are some locking issue here, to prevent application > > to send request and insert keys in LSM tree, no matter in writeback or > > writethrough mode. This is a lazy and fast response, I need to check the > > code then provide an accurate reply :-) > > What controls the frequency? > > On our system I noticed this: > > ]# tail /sys/block/bcache0/bcache/cache/internal/*gc* > > ==> /sys/block/bcache0/bcache/cache/internal/btree_gc_average_duration_ms <== > 1455 > > ==> /sys/block/bcache0/bcache/cache/internal/btree_gc_average_frequency_sec <== > 3521 > > ==> /sys/block/bcache0/bcache/cache/internal/btree_gc_max_duration_ms <== > 7036 > > > So usually it takes 1.4s, as much as 7s on our systems. Average frequency > is almost an hour. Can GC just be triggered more frequently? Say, once > every 5min? Is that configurable? > > -Eric > > > > > > > -- > > Coly Li > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > >
