On Wed, Apr 27 2016, Jens Axboe wrote:
> On Wed, Apr 27 2016, Jens Axboe wrote:
> > On 04/27/2016 12:01 PM, Jan Kara wrote:
> > >Hi,
> > >
> > >On Tue 26-04-16 09:55:23, Jens Axboe wrote:
> > >>Since the dawn of time, our background buffered writeback has sucked.
> > >>When we do background buffered writeback, it should have little impact
> > >>on foreground activity. That's the definition of background activity...
> > >>But for as long as I can remember, heavy buffered writers have not
> > >>behaved like that. For instance, if I do something like this:
> > >>
> > >>$ dd if=/dev/zero of=foo bs=1M count=10k
> > >>
> > >>on my laptop, and then try and start chrome, it basically won't start
> > >>before the buffered writeback is done. Or, for server oriented
> > >>workloads, where installation of a big RPM (or similar) adversely
> > >>impacts database reads or sync writes. When that happens, I get people
> > >>yelling at me.
> > >>
> > >>I have posted plenty of results previously, I'll keep it shorter
> > >>this time. Here's a run on my laptop, using read-to-pipe-async for
> > >>reading a 5g file, and rewriting it. You can find this test program
> > >>in the fio git repo.
> > >
> > >I have tested your patchset on my test system. Generally I have observed
> > >noticeable drop in average throughput for heavy background writes without
> > >any other disk activity and also somewhat increased variance in the
> > >runtimes. It is most visible on this simple testcases:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >
> > >and
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >
> > >The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
> > >created before each dd run on a dedicated disk.
> > >
> > >Without your patches I get pretty stable dd runtimes for both cases:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 87.9611 87.3279 87.2554
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 93.3502 93.2086 93.541
> > >
> > >With your patches the numbers look like:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 108.183, 97.184, 99.9587
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 104.9, 102.775, 102.892
> > >
> > >I have checked whether the variance is due to some interaction with CFQ
> > >which is used for the disk. When I switched the disk to deadline, I still
> > >get some variance although, the throughput is still ~10% lower:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 100.417 100.643 100.866
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 104.208 106.341 105.483
> > >
> > >The disk is rotational SATA drive with writeback cache, queue depth of the
> > >disk reported in /sys/block/sdb/device/queue_depth is 1.
> > >
> > >So I think we still need some tweaking on the low end of the storage
> > >spectrum so that we don't lose 10% of throughput for simple cases like
> > >this.
> >
> > Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
> > you are seeing smaller requests, and that is why it both varies and
> > you get lower throughput? I'll try and setup a test here similar to
> > yours.
>
> Jan, care to try the below patch? I can't fully reproduce your issue on
> a SCSI disk limited to QD=1, but I have a feeling this might help. It's
> a bit of a hack, but the general idea is to allow one more request to
> build up for QD=1 devices. That eliminates wait time between one request
> finishing, and the next being submitted.
That accidentally added a potentially stall, this one is both cleaner
and should have that fixed.
diff --git a/lib/wbt.c b/lib/wbt.c
index 650da911f24f..322f5e04e994 100644
--- a/lib/wbt.c
+++ b/lib/wbt.c
@@ -98,18 +98,23 @@ void __wbt_done(struct rq_wb *rwb)
else
limit = rwb->wb_normal;
+ inflight = atomic_dec_return(&rwb->inflight);
+
/*
- * Don't wake anyone up if we are above the normal limit. If
- * throttling got disabled (limit == 0) with waiters, ensure
- * that we wake them up.
+ * wbt got disabled with IO in flight. Wake up any potential
+ * waiters, we don't have to do more than that.
*/
- inflight = atomic_dec_return(&rwb->inflight);
- if (limit && inflight >= limit) {
- if (!rwb->wb_max)
- wake_up_all(&rwb->wait);
+ if (!rwb_enabled(rwb)) {
+ wake_up_all(&rwb->wait);
return;
}
+ /*
+ * Don't wake anyone up if we are above the normal limit.
+ */
+ if (inflight && inflight >= limit)
+ return;
+
if (waitqueue_active(&rwb->wait)) {
int diff = limit - inflight;
@@ -150,14 +155,26 @@ static void calc_wb_limits(struct rq_wb *rwb)
return;
}
- depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
-
/*
- * Reduce max depth by 50%, and re-calculate normal/bg based on that
+ * For QD=1 devices, this is a special case. It's important for those
+ * to have one request ready when one completes, so force a depth of
+ * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
+ * since the device can't have more than that in flight.
*/
- rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
- rwb->wb_normal = (rwb->wb_max + 1) / 2;
- rwb->wb_background = (rwb->wb_max + 3) / 4;
+ if (rwb->queue_depth == 1) {
+ rwb->wb_max = rwb->wb_normal = 2;
+ rwb->wb_background = 1;
+ } else {
+ depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
+
+ /*
+ * Reduce max depth by 50%, and re-calculate normal/bg based on
+ * that.
+ */
+ rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
+ rwb->wb_normal = (rwb->wb_max + 1) / 2;
+ rwb->wb_background = (rwb->wb_max + 3) / 4;
+ }
}
static bool inline stat_sample_valid(struct blk_rq_stat *stat)
--
Jens Axboe
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