Re: ext4 extent status tree LRU locking

From: Zheng Liu
Date: Wed Jun 12 2013 - 11:45:24 EST

On Wed, Jun 12, 2013 at 08:09:14AM -0700, Dave Hansen wrote:
> On 06/12/2013 12:17 AM, Zheng Liu wrote:
> > On Tue, Jun 11, 2013 at 04:22:16PM -0700, Dave Hansen wrote:
> >> I've got a test case which I intended to use to stress the VM a bit. It
> >> fills memory up with page cache a couple of times. It essentially runs
> >> 30 or so cp's in parallel.
> >
> > Could you please share your test case with me? I am glad to look at it
> > and think about how to improve LRU locking.
> I'll look in to giving you the actual test case. But I'm not sure of
> the licensing on it.

That would be great if you could share it.

> Essentially, the test creates an (small (~256MB) ext4 fs on a
> loopback-mounted ramfs device. It then goes and creates 160 64GB sparse
> files (one per cpu) and then cp's them all to /dev/null.

Thanks for letting me know.

> >> 98% of my CPU is system time, and 96% of _that_ is being spent on the
> >> spinlock in ext4_es_lru_add(). I think the LRU list head and its lock
> >> end up being *REALLY* hot cachelines and are *the* bottleneck on this
> >> test. Note that this is _before_ we go in to reclaim and actually start
> >> calling in to the shrinker. There is zero memory pressure in this test.
> >>
> >> I'm not sure the benefits of having a proper in-order LRU during reclaim
> >> outweigh such a drastic downside for the common case.
> >
> > A proper in-order LRU can help us to reclaim some memory from extent
> > status tree when we are under heavy memory pressure. When shrinker
> > tries to reclaim extents from these trees, some extents of files that
> > are accessed infrequnetly will be reclaimed because we hope that
> > frequently accessed files' extents can be kept in memory as much as
> > possible. That is why we need a proper in-order LRU list.
> Does it need to be _strictly_ in order, though? In other words, do you
> truly need a *global*, perfectly in-order LRU?
> You could make per-cpu LRUs, and batch movement on and off the global
> LRU once the local ones get to be a certain size. Or, you could keep
> them cpu-local *until* the shrinker is called, when the shrinker could
> go drain all the percpu ones.
> Or, you could tag each extent in memory with its last-used time. You
> write an algorithm to go and walk the tree and attempt to _generally_
> free the oldest objects out of a limited window.

Thanks for your suggestions. I will try these solutions, and look at
which one is best for us.

- Zheng
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