Thanks Coly! Can garbage collection be turned off, by echo 1 into /sys/fs/bcache/<UUID>/internal/gc_after_writeback ? The issue I'm seeing is, garbage collection causes write performance (writeback mode) to drop whenever the cache gets 50% full. With a 10gb cache device, an 8 GB write (using fio randwrite) should give SSD like speed, but it does not. I am wondering if its due to the gc threads. Regards, Nikhil. On Sat, 25 Jun 2022 at 17:38, Coly Li <colyli@xxxxxxx> wrote: > > > > > 2022年6月25日 14:29，Nikhil Kshirsagar <nkshirsagar@xxxxxxxxx> 写道： > > > > Hello, > > > > I've been doing some performance tests of bcache on 5.15.0-40-generic. > > > > The baseline figures for the fast and slow disk for random writes are > > consistent at around 225MiB/s and 3046KiB/s. > > > > But the bcache results inexplicably drop sometimes to 10Mib/s, for > > random write test using fio like this - > > > > fio --rw=randwrite --size=1G --ioengine=libaio --direct=1 > > --gtod_reduce=1 --iodepth=128 --bs=4k --name=MY_TEST1 > > > > WRITE: bw=168MiB/s (176MB/s), 168MiB/s-168MiB/s (176MB/s-176MB/s), > > io=1024MiB (1074MB), run=6104-6104msec > > WRITE: bw=283MiB/s (297MB/s), 283MiB/s-283MiB/s (297MB/s-297MB/s), > > io=1024MiB (1074MB), run=3621-3621msec > > WRITE: bw=10.3MiB/s (10.9MB/s), 10.3MiB/s-10.3MiB/s > > (10.9MB/s-10.9MB/s), io=1024MiB (1074MB), run=98945-98945msec > > WRITE: bw=8236KiB/s (8434kB/s), 8236KiB/s-8236KiB/s > > (8434kB/s-8434kB/s), io=1024MiB (1074MB), run=127317-127317msec > > WRITE: bw=9657KiB/s (9888kB/s), 9657KiB/s-9657KiB/s > > (9888kB/s-9888kB/s), io=1024MiB (1074MB), run=108587-108587msec > > WRITE: bw=4543KiB/s (4652kB/s), 4543KiB/s-4543KiB/s > > (4652kB/s-4652kB/s), io=1024MiB (1074MB), run=230819-230819msec > > > > This seems to happen after 2 runs of 1gb writes (cache disk is 4gb size) > > > > Some details are here - https://pastebin.com/V9mpLCbY , I will share > > the full testing results soon, but just was wondering about this > > performance drop for no apparent reason once the cache gets about 50% > > full. > > > It seems you are stuck by garbage collection. 4GB cache is small, the garbage collection might be invoked quite frequently. Maybe you can see the output of ’top -H’ to check whether there is kernel thread named bache_gc. > > Anyway, 4GB cache is too small. > > Coly Li >
