Re: RFC: one-bit mutexes (was: Re: [PATCH 2/3] Memory management livelock)

From: Nick Piggin
Date: Mon Oct 20 2008 - 21:51:32 EST

On Tuesday 21 October 2008 07:14, Mikulas Patocka wrote:
> > > If you are concerned about the size of an inode, I can convert other
> > > mutexes to bit mutexes: i_mutex and inotify_mutex.
> >
> > I wouldn't worry for now. mutexes can be unlocked much faster than bit
> > mutexes, especially in the fastpath. And due to slab, it would be
> > unlikely to actually save any space.
> Maybe inotify_mutex. You are right that i_mutex is so heavily contended
> that slowing it down to save few words wouldn't be good. Do you know about
> any inotify-intensive workload?

Don't really know, no. I think most desktop environments use it to
some extent, but no idea how much.

> > > I could also create
> > > bit_spinlock (one-bit spinlock that uses test_and_set_bit) and save
> > > space for address_space->tree_lock, address_space->i_mmap_lock,
> > > address_space->private_lock, inode->i_lock.
> >
> > We have that already. It is much much faster to unlock spinlocks than
> > bit spinlocks in general (if you own the word exclusively, then it's
> > not, but then you would be less likely to save space), and we can also
> > do proper FIFO ticket locks with a larger word.
> BTW. why do spinlocks on x86(64) have 32 bits and not 8 bits or 16 bits?
> Are atomic 32-bit instuctions faster?

In the case of <= 256 CPUs, they could be an unsigned short I think.
Probably it has never been found to be a huge win because they are
often beside other ints or longs. I think I actually booted up the
kernel with 16-bit spinlocks when doing the FIFO locks, but never
sent a patch for it... Don't let me stop you from trying though.

> Can x86(86) system have 256 CPUs?

Well, none that I know of which actually exist. SGI is hoping to have
4096 CPU x86 systems as far as I can tell.
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