dirty_ratio per device
Q: Are there any ways to "whitelist" the fast devices to have more write cache? Or to have the slow devices (or remote "devices" like //cifs/paths) use less write cache?
There are some settings for this, but they are not as effective as you hoped for. See the bdi ("backing device") objects in sysfs:
linux-4.18/Documentation/ABI/testing/sysfs-class-bdi
min_ratio (read-write)
Under normal circumstances each device is given a part of the total
write-back cache that relates to its current average writeout speed
in relation to the other devices.
The 'min_ratio' parameter allows assigning a minimum percentage of
the write-back cache to a particular device. For example, this is
useful for providing a minimum QoS.
max_ratio (read-write)
Allows limiting a particular device to use not more than the given
percentage of the write-back cache. This is useful in situations
where we want to avoid one device taking all or most of the
write-back cache. For example in case of an NFS mount that is prone
to get stuck.
The catch is "this setting only takes effect after
we have more than (dirty_background_ratio+dirty_ratio)/2 dirty data in
total. Because that is the amount of dirty data when we start to throttle
processes. So if the device you'd like to limit is the only one which is
currently written to, the limiting doesn't have a big effect." Further reading:
- LKML post by Jan Kara (2013).
- The "test case", at the end of this answer.
- commit 5fce25a9df48 in v2.6.24. "We allow violation of bdi limits if there is a lot of room on the system. Once we hit half the total limit we start enforcing bdi limits..." This is part of the same kernel release that added the internal per-device "limits". So the "limits" have always worked like this, except for pre-releases v2.6.24-rc1 and -rc2.
For simplicity, let us ignore your 30% setting and assume the defaults: dirty_background_ratio=10 and dirty_ratio=20. In this case, processes are allowed to dirty pages without any delays, until the system as a whole reaches the 15% point.
Q: The situation is reproducible; setting dirty_ratio = 1 solves the problem completely.
:-/
This sounds similar to the "pernicious USB-stick stall problem", which LWN.net wrote an article about. Unfortunately this particular article is misleading. It was so confused that it fabricated a different problem from the one that was reported.
One possibility is that you are reproducing a more specific defect. If you can report it to kernel developers, they might be able to analyze it and find a solution. Like the interaction with transparent hugepages was solved. You would be expected to reproduce the problem using the upstream kernel. Or talk to your paid support contact :).
Otherwise, there is a patch that can be applied to expose the internal strictlimit setting. This lets you change max_ratio into a strict limit. The patch has not been applied to mainline. If enough people show a need for this, the patch might get applied, or it might encourage some work to remove the need for it.
My concern is that while potentially useful, the feature
might not be sufficiently useful to justify its inclusion. So we'll
end up addressing these issues by other means, then we're left
maintaining this obsolete legacy feature.
I'm thinking that unless someone can show that this is good and
complete and sufficient for a "large enough" set of issues, I'll take a
pass on the patch[1]. What do people think?
[1] Actually, I'll stick it in -mm and maintain it, so next time
someone reports an issue I can say "hey, try this".
-- Andrew Morton, 2013
mm-add-strictlimit-knob-v2.patch is still sitting in -mm. A couple of times, people mentioned ideas about better auto-tuning the dirty cache. I haven't found a lot of work on it though. An appealing suggestion is to keep 5 seconds worth of write-back cache per device. However the speed of a device can change suddenly, e.g. depending whether the IO pattern is random or sequential.
Analysis (but no conclusion)
Q: I was flabbergasted to find out that kernel treated flushing pages to some slow remote CIFS box in exactly the same way as to super-fast local SSD drive.
These are not treated exactly the same. See the quote from the BDI doc above. "Each device is given a part of the total write-back cache that relates to its current average writeout speed."
However, this still makes it possible for the slow device to fill up the overall write-back cache, to somewhere between the 15-20% marks, if the slow device is the only one being written to.
If you start writing to a device which has less than its allowed share of the maximum writeback cache, the "dirty throttling" code should make some allowances. This would let you use some of the remaining margin, and avoid having to wait for the slow device to make room for you.
The doc suggests min_ratio and max_ratio settings were added in case your device speeds vary unpredictably, including stalling while an NFS server is unavailable.
The problem is if the dirty throttling fails to control the slow device, and it manages to fill up to (or near) the 20% hard limit.
The dirty throttling code that we're interested in was reshaped in v3.2. For an introduction, see the LWN.net article "IO-less dirty throttling". Also, following the release, Fengguang Wu presented at LinuxCon Japan. His presentation slides are very detailed and informative.
The goal was to delegate all writeback for a BDI to a dedicated thread, to allow a much better pattern of IO. But they also had to change to a less direct throttling system. At best, this makes the code harder to reason about. It has been well-tested, but I'm not sure that it covers every possible operating regime.
In fact looking at v4.18, there is explicit fallback code for a more extreme version of your problem: when one BDI is completely non-responsive. It tries to make sure other BDI's can still make forward progress, but... they would be much more limited in how much writeback cache they can use. Performance would likely be degraded, even if there is only one writer.
Q: Under memory pressure, when system reclaimed most of the read cache, system stubbornly tried to flush&reclaim the dirty (write) cache. So the situation was a huge CPU iowait accompanied with an excellent local disk I/O completion time, a lot of processes in D uninterruptible wait and a totally unresponsive system. OOM killer never engaged, because there was free memory that system wasn't giving out. (I think there is also a bug with CIFS, that crawled the flushing to incredibly slow speeds. But nevermind that here.)
You mention your system was under memory pressure. This is one example of a case which could be very challenging. When "available" memory goes down, it can put pressure on the size of the write-back cache. "dirty_ratio" is actually a percentage of "available" memory, which means free memory + page cache.
This case was noticed during the original work. There is an attempt to mitigate it. It says that "the new dirty limits are not going to avoid throttling the light dirtiers, but could limit their sleep time to 200ms."
Test case for "max_ratio"
Set up a VM / laptop / whatever, which does not have an expensively large amount of RAM. Run dd if=/dev/zero bs=1M of=~/test, and watch the write cache with grep -E '^(Dirty:|Writeback:)' /proc/meminfo. You should see dirty+writeback settle around a "set point".
The set point is 17.5%, half-way between 15% and 20%. My results on Linux v4.18 are here. If you want to see an exact percentage, be aware that the ratios are not a percentage of total RAM; I suggest you use the tracepoint in dirty_balance_pages().
I ran this test with different values of max_ratio in the filesystem's BDI. As expected, it was not possible to limit the write-back cache below the 15% point.
The problem is the "USB-stick stall" article provides no evidence for its claim. There have been genuine "USB-stick stall" problems, and there continue to be some similar reports. However the thread discussed by the LWN article is not one of them! Therefore we cannot cite the article as an example. Additionally, any explanations it gives must be flawed, or at least incomplete.
Why were "USB-stick stall" problems reported in 2013? Why wasn't this problem solved by the existing "No-I/O dirty throttling" code?
To summarize the linked answer:
The problem reported to linux-kernel did not see the entire system hang, while it was flushing cached writes to a USB stick. The initial report by Artem simply complained that Linux allowed a very large amount of cached writes on a slow device, which could take up to "dozens of minutes" before they finished.
As you say, Linus' suggested "fix" has not been applied. Current kernel versions (v4.20 and below) still allow systems with large RAM to build up large amounts writes in the page cache, which can take a long time to write out.
The kernel already had some code designed to avoid "USB-stick stalls". This is the "No-I/O dirty throttling" code. This code was also described on LWN, in 2011. It throttles write() calls to control both the size of the overall writeback cache, and the proportion of writeback cache used for the specific backing device. This is a complex engineered system, which has been tweaked over time. I am sure it will have some limitations. So far I am not able to quantify any limitation. There have also been various bugfixes outside the dirty throttling code, for issues which prevented it from being able to work.
WBT limits the number of submitted IO requests for each individual device. It does not limit the writeback cache, i.e. the dirty page cache.
Artem posted a followup report that writing 10GB to a server's internal disk caused the system to hang, or at least suffer extremely long delays in responding. That is consistent with the problem that WBT aims to address.
Sidenotes kept from previous versions of this answer:
The scenario described for WBT is when you are writing a large batch of data to your main disk, and at the same time you want to keep using your main disk interactively, to load programs etc.
In contrast, when people talk about a "USB-stick stall" problem, they mean writing a large batch of data to a different disk / external USB etc, and then suffering surprising delays in programs that have nothing to do with that disk. Example:
"Even things as simple as moving windows around could stutter... It wasn't CPU load, because ssh sessions to remote machines were perfectly responsive; instead it seemed that anything that might vaguely come near doing filesystem IO was extensively delayed."
The 2013 mailing list thread about the USB stick problem, mentioned per-device limits on dirty page cache as a possibility for future work.
WBT does not work with the CFQ or BFQ IO schedulers. Debian and Fedora use CFQ by default, so WBT will not help for USB sticks (nor spinning hard drives) unless you have some special configuration.
Traditionally CFQ has been used to work well with spinning hard drives. I'm not entirely sure where this leaves WBT. Maybe the main advantage of WBT is for SSDs, which are faster than spinning hard drives, but too slow to treat like RAM?
Or maybe it's an argument to use the deadline scheduler instead, and forgo the CFQ features. Ubuntu switched to deadline in version 14.04, but switched back to CFQ since version 17.04 (zesty). (I think CentOS 7.0 is too old to have WBT, but it claims to use CFQ for SATA drives, and deadline for all other drives. CentOS 7.0 also supports NVMe drives, but only shows "none" for their scheduler.)
Best Answer
Look at Documentation/sysctl/vm.txt:
The available memory is calculated in global_dirtyable_memory(). It is equal to the amount of free memory plus the page cache. It does not include swappable pages (i.e. anonymous memory allocations, memory which is not backed by a file).
This behaviour applies since Linux 3.14 (2014). Before this change, swappable pages were included in the global_dirtyable_memory() total.
Example statistics while running the
ddcommand:The last line shows about 3,150,000 kB "available" memory, and a total of 562,000 kB data either being written back or waiting for writeback. That makes it 17.8%. Although it seemed the proportion fluctuated above and below that level, and was more often closer to 15%. EDIT: although these figures look closer, please do not trust this method. It is still not the right calculation and it could give very wrong results. See the followup here.
I found this the hard way:
I noticed there is a tracepoint in balance_dirty_pages(), which can be used for "analyzing the dynamics of the throttling algorithms". So I used
perf:It showed that
dirty(measured in 4096-byte pages) was lower than I expected, becausesetpointwas low. I traced the code; it meant there must be a similarly low value forfreerunin the tracepoint definition, which is set to(thresh + bg_thresh) / 2... and worked my way back toglobal_dirtyable_memory().