On Mon, 5 Feb 2018, Jens Axboe wrote:
> On 2/5/18 12:22 PM, Jens Axboe wrote:
> > On 2/5/18 12:11 PM, Mikulas Patocka wrote:
> >> I have a workload where one process sends many asynchronous write bios
> >> (without waiting for them) and another process sends synchronous flush
> >> bios. During this workload, writeback throttling throttles down to one
> >> outstanding bio, and this incorrect throttling causes performance
> >> degradation (all write bios sleep in __wbt_wait and they couldn't be sent
> >> in parallel).
> >>
> >> The reason for this throttling is that wbt_data_dir counts flush requests
> >> in the read bucket. The flush requests (that take quite a long time)
> >> trigger this condition repeatedly:
> >> if (stat[READ].min > rwb->min_lat_nsec)
> >> and that condition causes scale down to one outstanding request, despite
> >> the fact that there are no read bios at all.
> >>
> >> A similar problem could also show up with REQ_OP_ZONE_REPORT and
> >> REQ_OP_ZONE_RESET - they are also counted in the read bucket.
> >>
> >> This patch fixes the function wbt_data_dir, so that only REQ_OP_READ
> >> requests are counted in the read bucket. The patch improves SATA
> >> write+flush throughput from 130MB/s to 350MB/s.
> >
> > Good catch. But I do wonder if we should account them at all. It
> > might make sense to account flushes as writes, but we really
> > should not account anything that isn't regular read/write IO
> > related.
> >
> > Something like the below? Potentially including REQ_OP_FLUSH in the
> > write latencies as well.
>
> Thinking about it, flush should just be counted as writes, and the
> rest disregarded. Ala below.
>
>
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index ae8de9780085..f92fc84b5e2c 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -697,7 +697,15 @@ u64 wbt_default_latency_nsec(struct request_queue *q)
>
> static int wbt_data_dir(const struct request *rq)
> {
> - return rq_data_dir(rq);
> + const int op = req_op(rq);
> +
> + if (op == REQ_OP_READ)
> + return READ;
> + else if (op == REQ_OP_WRITE || op == REQ_OP_FLUSH)
> + return WRITE;
> +
> + /* don't account */
> + return -1;
> }
>
> int wbt_init(struct request_queue *q)
>
> --
> Jens Axboe
Yes, this works.
BTW. write throttling still causes some performance degradation (350MB/s
vs. 500MB/s without throttling) - should there be some logic that disables
write throttling if no sync requests are received?
Something like - if there's sync read, enable write throttling - if there
were no sync reads in the last 5 seconds, disable write throttling?
Mikulas
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