On 04/04/2017 06:31 AM, Jan Kara wrote: > Writeback throttling does not play well with CFQ since that also tries > to throttle async writes. As a result async writeback can get starved in > presence of readers. As an example take a benchmark simulating > postgreSQL database running over a standard rotating SATA drive. There > are 16 processes doing random reads from a huge file (2*machine memory), > 1 process doing random writes to the huge file and calling fsync once > per 50000 writes and 1 process doing sequential 8k writes to a > relatively small file wrapping around at the end of the file and calling > fsync every 5 writes. Under this load read latency easily exceeds the > target latency of 75 ms (just because there are so many reads happening > against a relatively slow disk) and thus writeback is throttled to a > point where only 1 write request is allowed at a time. Thanks Jan, it's probably really hard to ever combine anything with CFQ, since it does its own thing. I have applied the patch. -- Jens Axboe
