Re: [PATCHSET][RFC] Make background writeback not suck

From: Jens Axboe
Date: Tue Mar 22 2016 - 18:04:07 EST

On 03/22/2016 03:51 PM, Dave Chinner wrote:
On Tue, Mar 22, 2016 at 11:55:14AM -0600, Jens Axboe wrote:
This patchset isn't as much a final solution, as it's demonstration
of what I believe is a huge issue. 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 has not behaved like
that.

Of course not. The IO scheduler is supposed to determine how we
meter out bulk vs latency sensitive IO that is queued. That's what
all the things like anticipatory scheduling for read requests was
supposed to address....

I'm guessing you're seeing problems like this because blk-mq has no
IO scheduler infrastructure and so no way of prioritising,
scheduling and/or throttling different types of IO? Would that be
accurate?

It's not just that, but obviously the IO scheduler would be one place to throttle it. This, in a way, is a way of scheduling the writeback writes better. But most of the reports I get on writeback sucking is not using scsi/blk-mq, they end up being "classic" on things like deadline.

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 data
base reads. When that happens, I get people yelling at me.

A quick demonstration - a fio job that reads a a file, while someone
else issues the above 'dd'. Run on a flash device, using XFS. The
vmstat output looks something like this:

--io---- -system-- ------cpu-----
bi bo in cs us sy id wa st
156 4648 58 151 0 1 98 1 0
0 0 64 83 0 0 100 0 0
0 32 76 119 0 0 100 0 0
26616 0 7574 13907 7 0 91 2 0
41992 0 10811 21395 0 2 95 3 0
46040 0 11836 23395 0 3 94 3 0
19376 1310736 5894 10080 0 4 93 3 0
116 1974296 1858 455 0 4 93 3 0
124 2020372 1964 545 0 4 92 4 0
112 1678356 1955 620 0 3 93 3 0
8560 405508 3759 4756 0 1 96 3 0
42496 0 10798 21566 0 0 97 3 0
42476 0 10788 21524 0 0 97 3 0

So writeback is running at about 2GB/s, meaning the memory is
cleaned in about 5s.

Correct, and at the same time destroying anything else that runs on the disk. For most use cases, not ideal. If we get in a tighter spot on memory or someone waits on it, yes, we should ramp up. But not for background cleaning.

The read starts out fine, but goes to shit when we start bacckground
flushing. The reader experiences latency spikes in the seconds range.
On flash.

With this set of patches applies, the situation looks like this instead:

--io---- -system-- ------cpu-----
bi bo in cs us sy id wa st
33544 0 8650 17204 0 1 97 2 0
42488 0 10856 21756 0 0 97 3 0
42032 0 10719 21384 0 0 97 3 0
42544 12 10838 21631 0 0 97 3 0
42620 0 10982 21727 0 3 95 3 0
46392 0 11923 23597 0 3 94 3 0
36268 512000 9907 20044 0 3 91 5 0
31572 696324 8840 18248 0 1 91 7 0
30748 626692 8617 17636 0 2 91 6 0
31016 618504 8679 17736 0 3 91 6 0
30612 648196 8625 17624 0 3 91 6 0
30992 650296 8738 17859 0 3 91 6 0
30680 604075 8614 17605 0 3 92 6 0
30592 595040 8572 17564 0 2 92 6 0
31836 539656 8819 17962 0 2 92 5 0

And now it runs at ~600MB/s, slowing down the rate at which memory
is cleaned by 60%.

Which is the point, correct... If we're not anywhere near being tight on memory AND nobody is waiting for this IO, then by definition, the foreground activity is the important one. For the case used here, that's the application doing reads.

Given that background writeback is relied on by memory reclaim to
clean memory faster than the LRUs are cycled, I suspect this is
going to have a big impact on low memory behaviour and balance,
which will then feed into IO breakdown problems caused by writeback
being driven from the LRUs rather than the flusher threads.....

You're missing the part where the intent is to only throttle it heavily when it's pure background writeback. Of course, if we are low on memory and doing reclaim, we should get much closer to device bandwidth.

If I run the above dd without the reader running, I'm already at 90% of the device bandwidth - not quite all the way there, since I still want to quickly be able to inject reads (or other IO) without having to wait for the queues to purge thousands of requests.

The above was the why. The how is basically throttling background
writeback. We still want to issue big writes from the vm side of things,
so we get nice and big extents on the file system end. But we don't need
to flood the device with THOUSANDS of requests for background writeback.
For most devices, we don't need a whole lot to get decent throughput.

Except, when the system is busy (e.g. CPU busy) and the writeback
threads can be starved of CPU by other operations, the writeback
queue depth needs to go way up so that we don't end up with idle
devices because the flusher threads are starved of CPU....

Sure, writeback always needs to make stable progress.

This adds some simple blk-wb code that keeps limits how much buffered
writeback we keep in flight on the device end. The default is pretty
low. If we end up switching to WB_SYNC_ALL, we up the limits. If the
dirtying task ends up being throttled in balance_dirty_pages(), we up
the limit. Currently there are tunables associated with this, see the
last patch for descriptions of those.

I welcome testing. The end goal here would be having much of this
auto-tuned, so that we don't lose substantial bandwidth for
background writes, while still maintaining decent non-wb
performance and latencies.

Right, another layer of "writeback tunables" is not really a
desirable outcome. We spent a lot of time making the dirty page
cache flushing not need tunables (i.e. via careful design of closed
loop feedback systems), so I think that if we're going to add a new
layer of throttling, we need to do the same thing. i.e. it needs to
adapt automatically and correctly to changing loads and workloads.

Fully agree, and that's what I stated as well. The current patchset is a way to experiment with improving background writeback, that's both in the very first paragraph of this email, and in the blk-wb.c file as well. I'm not a huge fan of tunables, nobody touches them, and we need to get it right out of the box.

I've already removed one set of tunables from this posting compared to what I had a week ago, it's moving in that direction.

--
Jens Axboe