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Date: Mon, 10 Dec 2012 12:39:45 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Mel Gorman <mgorman@...e.de>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Hugh Dickins <hughd@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Paul Turner <pjt@...gle.com>, Hillf Danton <dhillf@...il.com>,
David Rientjes <rientjes@...gle.com>,
Lee Schermerhorn <Lee.Schermerhorn@...com>,
Alex Shi <lkml.alex@...il.com>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/49] Automatic NUMA Balancing v10
* Mel Gorman <mgorman@...e.de> wrote:
> On Fri, Dec 07, 2012 at 12:01:13PM +0100, Ingo Molnar wrote:
> >
> > * Mel Gorman <mgorman@...e.de> wrote:
> >
> > > This is a full release of all the patches so apologies for the
> > > flood. [...]
> >
> > I have yet to process all your mails, but assuming I address all
> > your review feedback and the latest unified tree in tip:master
> > shows no regression in your testing, would you be willing to
> > start using it for ongoing work?
> >
>
> Ingo,
>
> If you had read the second paragraph of the mail you just responded to or
> the results at the end then you would have seen that I had problems with
> the performance. [...]
I've posted a (NUMA-placement sensitive workload centric)
performance comparisons between "balancenuma", AutoNUMA and
numa/core unified-v3 to:
https://lkml.org/lkml/2012/12/7/331
I tried to address all performance regressions you and others
have reported.
Here's the direct [bandwidth] comparison of 'balancenuma v10' to
my -v3 tree:
balancenuma | NUMA-tip
[test unit] : -v10 | -v3
------------------------------------------------------------
2x1-bw-process : 6.136 | 9.647: 57.2%
3x1-bw-process : 7.250 | 14.528: 100.4%
4x1-bw-process : 6.867 | 18.903: 175.3%
8x1-bw-process : 7.974 | 26.829: 236.5%
8x1-bw-process-NOTHP : 5.937 | 22.237: 274.5%
16x1-bw-process : 5.592 | 29.294: 423.9%
4x1-bw-thread : 13.598 | 19.290: 41.9%
8x1-bw-thread : 16.356 | 26.391: 61.4%
16x1-bw-thread : 24.608 | 29.557: 20.1%
32x1-bw-thread : 25.477 | 30.232: 18.7%
2x3-bw-thread : 8.785 | 15.327: 74.5%
4x4-bw-thread : 6.366 | 27.957: 339.2%
4x6-bw-thread : 6.287 | 27.877: 343.4%
4x8-bw-thread : 5.860 | 28.439: 385.3%
4x8-bw-thread-NOTHP : 6.167 | 25.067: 306.5%
3x3-bw-thread : 8.235 | 21.560: 161.8%
5x5-bw-thread : 5.762 | 26.081: 352.6%
2x16-bw-thread : 5.920 | 23.269: 293.1%
1x32-bw-thread : 5.828 | 18.985: 225.8%
numa02-bw : 29.054 | 31.431: 8.2%
numa02-bw-NOTHP : 27.064 | 29.104: 7.5%
numa01-bw-thread : 20.338 | 28.607: 40.7%
numa01-bw-thread-NOTHP : 18.528 | 21.119: 14.0%
------------------------------------------------------------
I also tried to reproduce and fix as many bugs you reported as
possible - but my point is that it would be _much_ better if we
actually joined forces.
> [...] You would also know that tip/master testing for the last
> week was failing due to a boot problem (issue was in mainline
> not tip and has been already fixed) and would have known that
> since the -v18 release that numacore was effectively disabled
> on my test machine.
I'm glad it's fixed.
> Clearly you are not reading the bug reports you are receiving
> and you're not seeing the small bit of review feedback or
> answering the review questions you have received either. Why
> would I be more forthcoming when I feel that it'll simply be
> ignored? [...]
I am reading the bug reports and addressing bugs as I can.
> [...] You simply assume that each batch of patches you place
> on top must be fixing all known regressions and ignoring any
> evidence to the contrary.
>
> If you had read my mail from last Tuesday you would even know
> which patch was causing the problem that effectively disabled
> numacore although not why. The comment about p->numa_faults
> was completely off the mark (long journey, was tired, assumed
> numa_faults was a counter and not a pointer which was
> careless). If you had called me on it then I would have
> spotted the actual problem sooner. The problem was indeed with
> the nr_cpus_allowed == num_online_cpus()s check which I had
> pointed out was a suspicious check although for different
> reasons. As it turns out, a printk() bodge showed that
> nr_cpus_allowed == 80 set in sched_init_smp() while
> num_online_cpus() == 48. This effectively disabling numacore.
> If you had responded to the bug report, this would likely have
> been found last Wednesday.
Does changing it from num_online_cpus() to num_possible_cpus()
help? (Can send a patch if you want.)
> > It would make it much easier for me to pick up your
> > enhancements, fixes, etc.
> >
> > > Changelog since V9
> > > o Migration scalability (mingo)
> >
> > To *really* see migration scalability bottlenecks you need to
> > remove the migration-bandwidth throttling kludge from your tree
> > (or configure it up very high if you want to do it simple).
> >
>
> Why is it a kludge? I already explained what the rational
> behind the rate limiting was. It's not about scalability, it's
> about mitigating worse-case behaviour and the amount of time
> the kernel spends moving data around which a deliberately
> adverse workload can trigger. It is unacceptable if during a
> phase change that a process would stall potentially for
> milliseconds (seconds if the node is large enough I guess)
> while the data is being migrated. Here is it again --
> http://www.spinics.net/lists/linux-mm/msg47440.html . You
> either ignored the mail or simply could not be bothered
> explaining why you thought this was the incorrect decision or
> why the concerns about an adverse workload were unimportant.
I think the stalls could have been at least in part due to the
scalability bottlenecks that the rate-limiting code has hidden.
If you think of the NUMA migration as a natural part of the
workload, as a sort of extended cache-miss, and if you assume
that the scheduler is intelligent about not flip-flopping tasks
between nodes (which the latest code certainly is), then I don't
see why the rate of migration should be rate-limited in the VM.
Note that I tried to quantify this effect: the perf bench numa
testcases start from a practical 'worst-case adverse' workload
in essence: all pages concentrated on the wrong node, and the
workload having to migrate all of them over.
We could add a new 'absolutely worst case' testcase, to make it
behaves sanely?
> I have a vague suspicion actually that when you are modelling
> the task->data relationship that you make an implicit
> assumption that moving data has zero or near-zero cost. In
> such a model it would always make sense to move quickly and
> immediately but in practice the cost of moving can exceed the
> performance benefit of accessing local data and lead to
> regressions. It becomes more pronounced if the nodes are not
> fully connected.
I make no such assumption - convergence costs were part of my
measurements.
> > Some (certainly not all) of the performance regressions you
> > reported were certainly due to numa/core code hitting the
> > migration codepaths as aggressively as the workload demanded
> > - and hitting scalability bottlenecks.
>
> How are you so certain? [...]
Hm, I don't think my "some (certainly not all)" statement
reflected any sort of certainty. So we violently agree about:
> [...] How do you not know it's because your code is migrating
> excessively for no good reason because the algorithm has a
> flaw in it? [...]
That's another source - but again not something we should fix by
hiding it under the carpet via migration bandwidth rate limits,
right?
> [...] Or that the cost of excessive migration is not being
> offset by local data accesses? [...]
That's another possibility.
The _real_ fix is to avoid excessive migration on the CPU and
memory placement side, not to throttle the basic mechanism
itself!
I don't exclude the possibility that bandwidth limits might be
needed - but only if everything else fails. Meanwhile, the
bandwidth limits were actively hiding scalability bottlenecks,
which bottlenecks only trigger at higher migration rates.
> [...] The critical point to note is that if it really was only
> scalability problems then autonuma would suffer the same
> problems and would be impossible to autonumas performance to
> exceed numacores. This isn't the case making it unlikely the
> scalability is your only problem.
The scheduling patterns are different - so they can hit
different bottlenecks.
> Either way, last night I applied a patch on top of latest
> tip/master to remove the nr_cpus_allowed check so that
> numacore would be enabled again and tested that. In some
> places it has indeed much improved. In others it is still
> regressing badly and in two case, it's corrupting memory --
> specjbb when THP is enabled crashes when running for single or
> multiple JVMs. It is likely that a zero page is being inserted
> due to a race with migration and causes the JVM to throw a
> null pointer exception. Here is the comparison on the rough
> off-chance you actually read it this time.
Can you still see the JVM crash with the unified -v3 tree?
Thanks,
Ingo
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