Re: [PATCH v4 0/9] mm/swap: Regular page swap optimizations

[Minchan Kim <minchan@kernel.org>]

Hi Huang,

On Wed, Dec 28, 2016 at 09:54:27AM +0800, Huang, Ying wrote:

< snip >

> > The patchset has used several techniqueus to reduce lock contention, for example,
> > batching alloc/free, fine-grained lock and cluster distribution to avoid cache
> > false-sharing. Each items has different complexity and benefits so could you
> > show the number for each step of pathchset? It would be better to include the
> > nubmer in each description. It helps how the patch is important when we consider
> > complexitiy of the patch.
> 
> One common problem of scalability optimization is that, after you have
> optimized one lock, the end result may be not very good, because another
> lock becomes heavily contended.  Similar problem occurs here, there are
> mainly two locks during swap out/in, one protects swap cache, the other
> protects swap device.  We can achieve good scalability only after having
> optimized the two locks.

Yes. You can describe that situation into the description. For example,
"with this patch, we can watch less swap_lock contention with perf but
overall performance is not good because swap cache lock still is still
contended heavily like below data so next patch will solve the problem".

It will make patch's justficiation clear.

> 
> You cannot say that one patch is not important just because the test
> result for that single patch is not very good.  Because without that,
> the end result of the whole series will be not very good.

I know that but this patchset are lack of number too much to justify
each works. You can show just raw number itself of a techniqueue
although it is not huge benefit or even worse. You can explain the reason
why it was not good, which would be enough motivation for next patch.

Number itself wouldn't be important but justfication is really crucial
to review/merge patchset and number will help it a lot in especially
MM community.

> 
> >> 
> >> Patch 1 is a clean up patch.
> >
> > Could it be separated patch?
> >
> >> Patch 2 creates a lock per cluster, this gives us a more fine graind lock
> >>         that can be used for accessing swap_map, and not lock the whole
> >>         swap device
> >
> > I hope you make three steps to review easier. You can create some functions like
> > swap_map_lock and cluster_lock which are wrapper functions just hold swap_lock.
> > It doesn't change anything performance pov but it clearly shows what kinds of lock
> > we should use in specific context.
> >
> > Then, you can introduce more fine-graind lock in next patch and apply it into
> > those wrapper functions.
> >
> > And last patch, you can adjust cluster distribution to avoid false-sharing.
> > And the description should include how it's bad in testing so it's worth.
> >
> > Frankly speaking, although I'm huge user of bit_spin_lock(zram/zsmalloc
> > have used it heavily), I don't like swap subsystem uses it.
> > During zram development, it really hurts debugging due to losing lockdep.
> > The reason zram have used it is by size concern of embedded world but server
> > would be not critical so please consider trade-off of spinlock vs. bit_spin_lock.
> 
> There will be one struct swap_cluster_info for every 1MB swap space.
> So, for example, for 1TB swap space, the number of struct
> swap_cluster_info will be one million.  To reduce the RAM usage, we
> choose to use bit_spin_lock, otherwise, spinlock is better.  The code
> will be used by embedded, PC and server, so the RAM usage is important.

It seems you already increase swap_cluster_info 4 byte to support
bit_spin_lock.
Compared to that, how much memory does spin_lock increase?

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