> 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
