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

    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

    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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      Last update: 2007-09-12 09:15    [W:0.024 / U:5.300 seconds]
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