Re: [00/17] Large Blocksize Support V3

Peter Zijlstra
2007-04-28T08:06:51Z

On Sat, 2007-04-28 at 11:43 +1000, Nick Piggin wrote:
> Andrew Morton wrote:

> > For example, see __do_page_cache_readahead(). It does a read_lock() and a
> > page allocation and a radix-tree lookup for each page. We can vastly
> > improve that.
> >
> > Step 1:
> >
> > - do a read-lock
> >
> > - do a radix-tree walk to work out how many pages are missing
> >
> > - read-unlock
> >
> > - allocate that many pages
> >
> > - read_lock()
> >
> > - populate all the pages.
> >
> > - read_unlock
> >
> > - if any pages are left over, free them
> >
> > - if we ended up not having enough pages, redo the whole thing.
> >
> > that will reduce the number of read_lock()s, read_unlock()s and radix-tree
> > descents by a factor of 32 or so in this testcase. That's a lot, and it's
> > something we (Nick ;)) should have done ages ago.
>
> We can do pretty well with the lockless radix tree (that is already upstream)
> there. I split that stuff out of my most recent lockless pagecache patchset,
> because it doesn't require the "scary" speculative refcount stuff of the
> lockless pagecache proper. Subject: [patch 5/9] mm: lockless probe.
>
> So that is something we could merge pretty soon.
>
> The other thing is that we can batch up pagecache page insertions for bulk
> writes as well (that is. write(2) with buffer size > page size). I should
> have a patch somewhere for that as well if anyone interested.

Together with the optimistic locking from my concurrent pagecache that
should bring most of the gains:

sequential insert of 8388608 items:

CONFIG_RADIX_TREE_CONCURRENT=n

[ffff81007d7f60c0] insert 0 done in 15286 ms

CONFIG_RADIX_TREE_OPTIMISTIC=y

[ffff81006b36e040] insert 0 done in 3443 ms

only 4.4 times faster, and more scalable, since we don't bounce the
upper level locks around.