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SubjectRe: [PATCH v4 0/9] mm/swap: Regular page swap optimizations
Date
Minchan Kim <minchan@kernel.org> writes:

> On Wed, Dec 28, 2016 at 11:31:06AM +0800, Huang, Ying wrote:
>
> < snip >
>
>> >>> > 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).
>
> Thanks for the information.
> To me, it's not big when we consider spinlock's usefullness which helps
> cache-line bouncing, lockdep and happy with RT people.

Yes. spinlock helps on lockdep and RT, but I don't think it helps
cache-line bouncing.

> So, I vote spin_lock but I'm not in charge of deciding on that and your
> opinion might be different still. If so, let's pass the decision to
> maintainer.

I have no strong opinion for size change on 32bit platform. But I want
to know other people's opinion, especially maintainer's too.

> Instead, please write down above content in description for maintainer to
> judge it fairly.

Sure.

Best Regards,
Huang, Ying

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