Re: vm lock contention reduction

[Alexander Viro <viro@math.psu.edu>]

On Thu, 4 Jul 2002, Linus Torvalds wrote:

> In particular, the ext2 superblock lock at least used to be horribly
> broken and held in a lot of "bad" places: I doubt Al has gotten far enough
> to fix that brokenness. The superblock lock used to cause one process that

As the matter of fact, I did.  If you want lock_super() to be killed in ext2
(2.5) - just say so and I'll do the rest.

Right now both ext2_new_block() and ext2_new_inode() look through the
group descriptors for good one and reserve (block|inode) in it.  That
can be easily done under a spinlock.  After that we read a bitmap
(no need for any locks) and grab a bit in it (we are guaranteed to
have one).  The latter can be either done under a spinlock or by being
clever and noticing that amount of contenders is always less or equal
the number of free bits (with minimal use of set_bit()/etc. atomicity
we can do that without spinlocks).  After that we don't need any locks
whatsoever.

Andrew had just killed the last bit of crap there - LRU used to be protected
by lock_super() and since it's no more...

Rewrite of balloc.c and ialloc.c was done with killing lock_super() in mind -
I didn't want to do that in 2.4 for obvious reasons, but for 2.5 it's very
easy...

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