Re: [PATCH v5 2/9] mm/swap: Add cluster lock

[Huang\, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Thu, 12 Jan 2017 09:47:51 +0800 "Huang\, Ying" <ying.huang@xxxxxxxxx> wrote:
>
>> >> > 1MB swap space, so for 1TB swap space, the total size will be 80M
>> >> > compared with 8M of current implementation.
>> >
>> > Where did this 80 bytes come from?  That swap_cluster_info is 12 bytes
>> > and could perhaps be squeezed into 8 bytes if we can get away with a
>> > 24-bit "count".
>> 
>> Sorry, I made a mistake when measuring the size of swap_cluster_info
>> when I sent that email, because I turned on the lockdep when measuring.
>> I have sent out a correction email to Jonathan when I realized that
>> later.
>> 
>> So the latest size measuring result is:
>> 
>> If we use bit_spin_lock, the size of cluster_swap_info will,
>> 
>> - increased from 4 bytes to 8 bytes on 64 bit platform
>> - keep as 4 bytes on 32 bit platform
>> 
>> If we use normal spinlock (queue spinlock), the size of cluster_swap_info will,
>> 
>> - increased from 4 bytes to 8 bytes on 64 bit platform
>> - increased from 4 bytes to 8 bytes on 32 bit platform
>> 
>> So the difference occurs on 32 bit platform.  If the size increment on
>> 32 bit platform is OK, then I think it should be good to use normal
>> spinlock instead of bit_spin_lock.  Personally, I am OK for that.  But I
>> don't know whether there will be some embedded world people don't like
>> it.
>
> I think that'll be OK - the difference is small and many small systems
> disable swap anyway.  So can we please try that?  Please do describe
> the additional overhead (with numbers) in the changelog: "additional
> bytes of RAM per GB of swap", for example.  And please also rerun the
> performance tests, see if we can notice the alleged speed improvements
> from switching to a spinlock.

Sure.  I will change it and redo the test.

Best Regards,
Huang, Ying