Re: [PATCH] cfq: Disable writeback throttling by default

[Jens Axboe <axboe@kernel.dk>]

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