Re: UMA cache back pressure

On 18.11.2013 21:11, Jeff Roberson wrote:
> On Mon, 18 Nov 2013, Alexander Motin wrote:
>> I've created patch, based on earlier work of avg_at_, to add back
>> pressure to UMA allocation caches. The problem of physical memory or
>> KVA exhaustion existed there for many years and it is quite critical
>> now for improving systems performance while keeping stability. Changes
>> done in memory allocation last years improved situation. but haven't
>> fixed completely. My patch solves remaining problems from two sides:
>> a) reducing bucket sizes every time system detects low memory
>> condition; and b) as last-resort mechanism for very low memory
>> condition, it cycling over all CPUs to purge their per-CPU UMA caches.
>> Benefit of this approach is in absence of any additional hard-coded
>> limits on cache sizes -- they are self-tuned, based on load and memory
>> pressure.
>>
>> With this change I believe it should be safe enough to enable UMA
>> allocation caches in ZFS via vfs.zfs.zio.use_uma tunable (at least for
>> amd64). I did many tests on machine with 24 logical cores (and as
>> result strong allocation cache effects), and can say that with 40GB
>> RAM using UMA caches, allowed by this change, by two times increases
>> results of SPEC NFS benchmark on ZFS pool of several SSDs. To test
>> system stability I've run the same test with physical memory limited
>> to just 2GB and system successfully survived that, and even showed
>> results 1.5 times better then with just last resort measures of b). In
>> both cases tools/umastat no longer shows unbound UMA cache growth,
>> that makes me believe in viability of this approach for longer runs.
>>
>> I would like to hear some comments about that:
>> http://people.freebsd.org/~mav/uma_pressure.patch
>
> Hey Mav,
>
> This is a great start and great results.  I think it could probably even
> go in as-is, but I have a few suggestions.

Hey! Thanks for your review. I appreciate.

> First, let's test this with something that is really super allocator
> heavy and doesn't benefit much from bucket sizing.  For example, a
> network forwarding test.  Or maybe you could get someone like Netflix
> that is using it to push a lot of bits with less filesystem cost than
> zfs and spec.

I am not sure what simple forwarding may show in this case. Even on my 
workload with ZFS creating strong memory pressure I still have mbuf* 
zones buckets almost (some totally) maxed out. Without other major (or 
even any) pressure in system they just can't become bigger then maximum. 
But if you can propose some interesting test case with pressure that I 
can reproduce -- I am all ears.

> Second, the cpu binding is a very costly and very high-latency
> operation. It would make sense to do CPU_FOREACH and then ZONE_FOREACH.
> You're also biasing the first zones in the list.  The low memory
> condition will more often clear after you check these first zones.  So
> you might just check it once and equally penalize all zones.  I'm
> concerned that doing CPU_FOREACH in every zone will slow the pagedaemon
> more.

I completely agree with all you said here. This part of code I just took 
as-is from earlier work. It definitely can be improved. I'll take a look 
on that. But as I have mentioned in one of earlier responses that code 
used in _very_ rare cases, unless system is heavily overloaded on 
memory, like doing ZFS on box with 24 cores and 2GB RAM. During 
reasonable operation it is enough to have soft back pressure to keep on 
caches in shape and never call that.

> We also have been working towards per-domain pagedaemons so
> perhaps we should have a uma-reclaim taskqueue that we wake up to do the
> work?

VM is not my area so far, so please propose "the right way". I took this 
task now only because I have to due to huge performance bottleneck this 
problem causes and years it remains unsolved.

> Third, using vm_page_count_min() will only trigger when the pageout
> daemon can't keep up with the free target.  Typically this should only
> happen with a lot of dirty mmap'd pages or incredibly high system load
> coupled with frequent allocations.  So there may be many cases where
> reclaiming the extra UMA memory is helpful but the pagedaemon can still
> keep up while pushing out file pages that we'd prefer to keep.

As I have told that is indeed last resort. It does not need to be done 
often. Per-CPU caches just should not grow without real need to the 
point when they have to be cleaned.

> I think the perfect heuristic would have some idea of how likely the UMA
> pages are to be re-used immediately so we can more effectively tradeoff
> between file pages and kernel memory cache.  As it is now we limit the
> uma_reclaim() calls to every 10 seconds when there is memory pressure.
> Perhaps we could keep a timestamp for when the last slab was allocated
> to a zone and do the more expensive reclaim on zones who have timestamps
> that exceed some threshold?  Then have a lower threshold for reclaiming
> at all? Again, it doesn't need to be perfect, but I believe we can catch
> a wider set of cases by carefully scheduling this.

I was thinking about that too. But I think timestamps should be set not 
on slab, but on bucket. The fact that zone is not allocating new slabs 
does not mean it does not use its already allocated buckets. If we put 
time of the last refill into each bucket, then we should be able to 
purge all buckets, unused for specified period of time. Additionally we 
could put timestamp on zone and update it every time zone runs out of 
its cache. If zone does not run out of cache for some time -- probably 
it has unused buckets. So when we need some RAM we should take a first 
look on zones that had stale timestamp.

-- 
Alexander Motin