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

["Huang\, Ying" <ying.huang@intel.com>]

"Huang, Ying" <ying.huang@intel.com> writes:

> Minchan Kim <minchan@kernel.org> writes:
>
>> 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.
>
> The increment only occurs on 64bit platform.  On 32bit platform, the
> size is the same as before.
>
>> Compared to that, how much memory does spin_lock increase?
>
> The size of struct swap_cluster_info will increase from 4 bytes to 16
> bytes on 64bit platform.  I guess it will increase from 4 bytes to 8
> bytes on 32bit platform at least, but I did not test that.

Sorry, I make a mistake during test.  The size of struct
swap_cluster_info will increase from 4 bytes to 8 bytes on 64 bit
platform.  I think it will increase from 4 bytes to 8 bytes on 32 bit
platform too (not tested).

Best Regards,
Huang, Ying