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

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Date:	Mon, 20 Oct 2008 16:14:15 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: RFC: one-bit mutexes (was: Re: [PATCH 2/3] Memory management
 livelock)

> > 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?

> > 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?

Can x86(86) system have 256 CPUs?

Mikulas
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