Message-ID: <56E9BCDF.7050905@hpe.com>
Date:	Wed, 16 Mar 2016 16:06:55 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
CC:	Dave Chinner <dchinner@...hat.com>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>, <xfs@....sgi.com>,
	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH 0/2] percpu_counter: Enable switching to global counter

On 03/07/2016 04:33 PM, Dave Chinner wrote:
> On Mon, Mar 07, 2016 at 12:39:55PM -0500, Waiman Long wrote:
>> On 03/05/2016 01:34 AM, Dave Chinner wrote:
>>> On Fri, Mar 04, 2016 at 09:51:37PM -0500, Waiman Long wrote:
>>>> This patchset allows the degeneration of per-cpu counters back
>>>> to global counters when:
>>>>
>>>>   1) The number of CPUs in the system is large, hence a high
>>>>   cost for calling percpu_counter_sum().  2) The initial count
>>>>   value is small so that it has a high chance of excessive
>>>>   percpu_counter_sum() calls.
>>>>
>>>> When the above 2 conditions are true, this patchset allows the
>>>> user of per-cpu counters to selectively degenerate them into
>>>> global counters with lock. This is done by calling the new
>>>> percpu_counter_set_limit() API after percpu_counter_set().
>>>> Without this call, there is no change in the behavior of the
>>>> per-cpu counters.
>>>>
>>>> Patch 1 implements the new percpu_counter_set_limit() API.
>>>>
>>>> Patch 2 modifies XFS to call the new API for the m_ifree and
>>>> m_fdblocks per-cpu counters.
>>>>
>>>> Waiman Long (2): percpu_counter: Allow falling back to global
>>>> counter on large system xfs: Allow degeneration of
>>>> m_fdblocks/m_ifree to global counters
>>> NACK.
>>>
>>> This change to turns off per-counter free block counters for 32p
>>> for the XFS free block counters.  We proved 10 years ago that a
>>> global lock for these counters was a massive scalability
>>> limitation for concurrent buffered writes on 16p machines.
>>>
>>> IOWs, this change is going to cause fast path concurrent
>>> sequential write regressions for just about everyone, even on
>>> empty filesystems.
>> That is not really the case here. The patch won't change anything
>> if there is enough free blocks available in the filesystem.  It
>> will turn on global lock at mount time iff the number of free
>> blocks available is less than the given limit. In the case of XFS,
>> it is 12MB per CPU. On the 80-thread system that I used for
>> testing, it will be a bit less than 1GB. Even if global lock is
>> enabled at the beginning, it will be transitioned back to percpu
>> lock as soon as enough free blocks become available.
> Again: How is this an optimisation that is generally useful? Nobody
> runs their production 80-thread workloads on a filesystems with less
> than 1GB of free space. This is a situation that most admins would
> consider "impending doom".

In most cases, there will be enough free blocks in m_fdblocks that the 
switching to global count will never happen. However, I found that 
m_ifree is a different story. On the 80-cpu system that I used, the 
percpu slowpath will be activated when there are less than 2*80^2 = 
12800 free inodes available which is usually the case because the code 
use the default batch size (which scale linearly with # of cpus). Here, 
my patch can really help.

>
>> I am aware that if there are enough threads pounding on the lock,
>> it can cause a scalability bottleneck. However, the qspinlock used
>> in x86 should greatly alleviate the scalability impact compared
>> with 10 years ago when we used the ticket lock.
> Regardless of whether there is less contention, it still brings back
> a global serialisation point and modified cacheline (the free block
> counter) in the filesystem that, at some point, will limit
> concurrency....

Yes, that is true, but the alternative here is to access all the 
cachelines of the percpu counters and evict quite a number of other 
useful cachelines along the way. My patch activates the global counter 
at mount time only when the current count is too small. It was proven in 
my test case that accessing all those cachelines was worse that taken 
the lock when there are large number of cpus.

Once the counter increase past the limit, it will disable the global 
counter and fall back to the usual per-cpu mode. The global counter 
won't be reactivated unless you unmount and remount the filesystem 
again. So I don't this case will cause any performance bottleneck that 
is worse than what the existing code is.

>> BTW, what exactly
>> was the microbenchmark that you used to exercise concurrent
>> sequential write? I would like to try it out on the new hardware
>> and kernel.
> Just something that HPC apps have been known to do for more then 20
> years: concurrent sequential write from every CPU in the system.
>
> http://oss.sgi.com/projects/xfs/papers/ols2006/ols-2006-paper.pdf

Thanks.

>
>>> near to ENOSPC. As i asked you last time - if you want to make
>>> this problem go away, please increase the size of the filesystem
>>> you are running your massively concurrent benchmarks on.
>>>
>>> IOWs, please stop trying to optimise a filesystem slow path that:
>>>
>>> 	a) 99.9% of production workloads never execute, b) where we
>>> 	expect performance to degrade as allocation gets
>>> 	computationally expensive as we close in on ENOSPC, c) we
>>> 	start to execute blocking data flush operations that slow
>>> 	everything down massively, and d) is indicative that the
>>> 	workload is about to suffer from a fatal, unrecoverable
>>> 	error (i.e. ENOSPC)
>>>
>> I totally agree. I am not trying to optimize a filesystem
>> slowpath.
> Where else in the kernel is there a requirement for 100%
> accurate threshold detection on per-cpu counters? There isn't, is
> there?

I don't quite get what you are asking here. The goal of this patch is 
not 100% accurate threshold detection. This is not what I am aiming for. 
Instead, I am trying to reduce the performance impact due to excessive 
per_counter_sum() calls. Accurate threshold detection is just a side 
effect, not the main purpose for this patch.

>> There are use cases, however, where we may want to
>> create relatively small filesystem. One example that I cited in
>> patch 2 is the battery backed NVDIMM that I have played with
>> recently. They can be used for log files or other small files.
>> Each dimm is 8 GB. You can have a few of those available. So the
>> filesystem size could be 32GB or so.  That can come close to the
>> the limit where excessive percpu_counter_sum() call can happen.
>> What I want to do here is to try to reduce the chance of excessive
>> percpu_counter_sum() calls causing a performance problem. For a
>> large filesystem that is nowhere near ENOSPC, my patch will have
>> no performance impact whatsoever.
> Yet your patch won't have any effect on these "small" filesystems
> because unless they have less free space than your threshold at
> mount time (rare!) they won't ever have this global lock turned on.
> Not to mention if space if freed in the fs, the global lock is
> turned off, and will never get turned back on.

Yes, as I said above, the m_fdblocks counter isn't an issue in almost 
all the cases. However, the patch was found to be useful in reducing 
performance overhead of the percpu_counter_sum() call on the m_ifree 
counter on a moderately sized XFS partition. BTW, I made no change in 
the XFS code other than activating the counter limits. If those limits 
are not activated, there is absolutely no change to the current behavior.

Yes, the benchmark that I shown in the changelog isn't real-life 
scenario. I will try to look for other benchmark results that are more 
realistic.

Cheers,
Longman


> Further, anyone using XFS on nvdimms will be enabling DAX, which
> goes through the direct IO path rather than the buffered IO path
> that is generating all this block accounting pressure. Hence it will
> behave differently, and so your solution doesn't obviously apply to
> that workload space, either.
>
> When we get production workloads hitting free block accounting
> issues near ENOSPC, then we'll look at optimising the XFS accounting
> code. Microbenchmarks are great when they have real-work relevance,
> but this doesn't right now. Not to mention we've got bigger things
> to worry about in XFS right now in terms of ENOSPC accounting (think
> reverse mapping, shared blocks and breaking shares via COW right
> next to ENOSPC) and getting these working *correctly* takes
> precendence of optimisation of the accounting code.
>

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