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SubjectRe: [RFC PATCH 1/1] vmscan: Support multiple kswapd threads per node

> On Apr 3, 2018, at 12:07 PM, Matthew Wilcox <> wrote:
> On Tue, Apr 03, 2018 at 03:31:15PM +0200, Michal Hocko wrote:
>> On Mon 02-04-18 09:24:22, Buddy Lumpkin wrote:
>>> The presence of direct reclaims 10 years ago was a fairly reliable
>>> indicator that too much was being asked of a Linux system. Kswapd was
>>> likely wasting time scanning pages that were ineligible for eviction.
>>> Adding RAM or reducing the working set size would usually make the problem
>>> go away. Since then hardware has evolved to bring a new struggle for
>>> kswapd. Storage speeds have increased by orders of magnitude while CPU
>>> clock speeds stayed the same or even slowed down in exchange for more
>>> cores per package. This presents a throughput problem for a single
>>> threaded kswapd that will get worse with each generation of new hardware.
>> AFAIR we used to scale the number of kswapd workers many years ago. It
>> just turned out to be not all that great. We have a kswapd reclaim
>> window for quite some time and that can allow to tune how much proactive
>> kswapd should be.
>> Also please note that the direct reclaim is a way to throttle overly
>> aggressive memory consumers. The more we do in the background context
>> the easier for them it will be to allocate faster. So I am not really
>> sure that more background threads will solve the underlying problem. It
>> is just a matter of memory hogs tunning to end in the very same
>> situtation AFAICS. Moreover the more they are going to allocate the more
>> less CPU time will _other_ (non-allocating) task get.
>>> Test Details
>> I will have to study this more to comment.
>> [...]
>>> By increasing the number of kswapd threads, throughput increased by ~50%
>>> while kernel mode CPU utilization decreased or stayed the same, likely due
>>> to a decrease in the number of parallel tasks at any given time doing page
>>> replacement.
>> Well, isn't that just an effect of more work being done on behalf of
>> other workload that might run along with your tests (and which doesn't
>> really need to allocate a lot of memory)? In other words how
>> does the patch behaves with a non-artificial mixed workloads?
>> Please note that I am not saying that we absolutely have to stick with the
>> current single-thread-per-node implementation but I would really like to
>> see more background on why we should be allowing heavy memory hogs to
>> allocate faster or how to prevent that. I would be also very interested
>> to see how to scale the number of threads based on how CPUs are utilized
>> by other workloads.
> Yes, very much this. If you have a single-threaded workload which is
> using the entirety of memory and would like to use even more, then it
> makes sense to use as many CPUs as necessary getting memory out of its
> way. If you have N CPUs and N-1 threads happily occupying themselves in
> their own reasonably-sized working sets with one monster process trying
> to use as much RAM as possible, then I'd be pretty unimpressed to see
> the N-1 well-behaved threads preempted by kswapd.

The default value provides one kswapd thread per NUMA node, the same
it was without the patch. Also, I would point out that just because you devote
more threads to kswapd, doesn’t mean they are busy. If multiple kswapd threads
are busy, they are almost certainly doing work that would have resulted in
direct reclaims, which are often substantially more expensive than a couple
extra context switches due to preemption.

Also, the code still uses wake_up_interruptible to wake kswapd threads, so
after starting the first kswapd thread, free pages minus the size of the allocation
would still need to be below the low watermark for a page allocation at that time
to cause another kswapd thread to wake up.

When I first decided to try this out, I figured a lot of tuning would be needed to
see good behavior. But what I found in practice was that it actually works quite
well. When you look closely, you see that there is very little difference between
a direct reclaim and kswapd. In fact, direct reclaims work a little harder than
kswapd, and they should continue to do so because that prevents the number
of parallel scanning tasks from increasing unnecessarily.

Please try it out, you might be surprised at how well it works.

> My biggest problem with the patch-as-presented is that it's yet one more
> thing for admins to get wrong. We should spawn more threads automatically
> if system conditions are right to do that.

I totally agree with this. In my previous response to Michal Hocko, I described
how I think we could scale watermarks in response to direct reclaims, and
launch more kswapd threads when kswapd peaks at 100% CPU usage.

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