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SubjectRe: [PATCH v5 2/9] mm/swap: Add cluster lock
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Hi, Andrew,

Andrew Morton <akpm@linux-foundation.org> writes:

> On Wed, 11 Jan 2017 16:07:29 -0700 Jonathan Corbet <corbet@lwn.net> wrote:
>
>> On Wed, 11 Jan 2017 15:00:29 -0800
>> Andrew Morton <akpm@linux-foundation.org> wrote:
>>
>> > hm, bit_spin_lock() is a nasty thing. It is slow and it doesn't have
>> > all the lockdep support.
>> >
>> > Would the world end if we added a spinlock to swap_cluster_info?
>>
>> FWIW, I asked the same question in December, this is what I got:
>>
>> ...
>>
>> > > Why the roll-your-own locking and data structures here? To my naive
>> > > understanding, it seems like you could do something like:
>> > >
>> > > struct swap_cluster_info {
>> > > spinlock_t lock;
>> > > atomic_t count;
>> > > unsigned int flags;
>> > > };
>> > >
>> > > Then you could use proper spinlock operations which, among other things,
>> > > would make the realtime folks happier. That might well help with the
>> > > cache-line sharing issues as well. Some of the count manipulations could
>> > > perhaps be done without the lock entirely; similarly, atomic bitops might
>> > > save you the locking for some of the flag tweaks - though I'd have to look
>> > > more closely to be really sure of that.
>> > >
>> > > The cost, of course, is the growth of this structure, but you've already
>> > > noted that the overhead isn't all that high; seems like it could be worth
>> > > it.
>> >
>> > Yes. The data structure you proposed is much easier to be used than the
>> > current one. The main concern is the RAM usage. The size of the data
>> > structure you proposed is about 80 bytes, while that of the current one
>> > is about 8 bytes. There will be one struct swap_cluster_info for every
>> > 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.

Best Regards,
Huang, Ying

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