Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Nick Piggin]

On Wednesday 12 September 2007 11:49, David Chinner wrote:
> On Tue, Sep 11, 2007 at 04:00:17PM +1000, Nick Piggin wrote:
> > > > OTOH, I'm not sure how much buy-in there was from the filesystems
> > > > guys. Particularly Christoph H and XFS (which is strange because they
> > > > already do vmapping in places).
> > >
> > > I think they use vmapping because they have to, not because they want
> > > to. They might be a lot happier with fsblock if it used contiguous
> > > pages for large blocks whenever possible - I don't know for sure. The
> > > metadata accessors they might be unhappy with because it's inconvenient
> > > but as Christoph Hellwig pointed out at VM/FS, the filesystems who
> > > really care will convert.
> >
> > Sure, they would rather not to. But there are also a lot of ways you can
> > improve vmap more than what XFS does (or probably what darwin does)
> > (more persistence for cached objects, and batched invalidates for
> > example).
>
> XFS already has persistence across the object life time (which can be many
> tens of seconds for a frequently used buffer)

But you don't do a very good job. When you go above 64 cached mappings,
you purge _all_ of them. fsblock's vmap cache can have a much higher number
(if you want), and purging can just unmap a batch which is decided by a simple
LRU (thus important metadata gets saved).


> and it also does batched 
> unmapping of objects as well.

It also could do a lot better at unmapping. Currently you're just calling
vunmap a lot of times in sequence. That still requires global IPIs and TLB
flushing every time.

This simple patch should easily be able to reduce that number by 2 or 3
orders of magnitude (maybe more on big systems).
http://www.mail-archive.com/linux-arch@xxxxxxxxxxxxxxx/msg03956.html

vmap area locking and data structures could also be made a lot better
quite easily, I suspect.


> > There are also a lot of trivial things you can do to make a lot of those
> > accesses not require vmaps (and less trivial things, but even such things
> > as binary searches over multiple pages should be quite possible with a
> > bit of logic).
>
> Yes, we already do the many of these things (via xfs_buf_offset()), but
> that is not good enough for something like a memcpy that spans multiple
> pages in a large block (think btree block compaction, splits and
> recombines).

fsblock_memcpy(fsblock *src, int soff, fsblock *dst, int doff, int size); ?


> IOWs, we already play these vmap harm-minimisation games in the places
> where we can, but still the overhead is high and something we'd prefer
> to be able to avoid.

I don't think you've looked nearly far enough with all this low hanging
fruit.

I just gave 4 things which combined might easily reduce xfs vmap overhead
by several orders of magnitude, all without changing much code at all.
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