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

[Andrew Morton <akpm@linux-foundation.org>]

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".


> > In the other hand, the return of the increased size is not overwhelming.
> > The bit spinlock on cluster will not be heavy contended because it is a
> > quite fine-grained lock.  So the benefit will be little to use lockless
> > operations.  I guess the realtime issue isn't serious given the lock is
> > not heavy contended and the operations protected by the lock is
> > light-weight too.

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