Re: 2.4.19pre1aa1

[Andrew Morton <akpm@zip.com.au>]

Andrea Arcangeli wrote:
> 
>
> depends what you're doing, if you do `cp /dev/zero .` and the fs is
> lucky enough to have free contigous space I definitely can see the
> improvement of highlevel merging, but that's not always what you're
> doing with the fs, for example that's not the case for kernel compiles
> and small files where you'll be always fragmented and where the bio will
> at max hold 4k and you keep rewriting into cache.

Cache effects.  We touch the buffers at prepare_write.  We touch them
again at commit_write().  And at writeout time.  And at page reclaim
time.  I think it's this general white-noise cost which is causing
the funny profiles which I'm seeing.  (For example, with no-buffers,
the cost of the IDE driver setup and interrupt handler has nosedived).

> The times you enter
> get_block you enter in a fs lock, rather than staying at the per-page
> lock, it's not additional locking, the bh on the pagecahce doesn't need
> any additional locking.

For writes, we have the lru list insertion, and the hashtable lock (twice).

> So for a kernel compile the current situation is
> an obvious advantage in performance and scalability (fs code definitely
> doesn't scale at the moment).

mm..  Delayed allocation means that the short-lived files never get
a disk mapping at all.

And yes, if all files are 100% fragmented then the BIO aggregation
doesn't help as much.

> But ok, globally it will be probably better to drop the bh since we have
> to work on the bio anyways somehow and so at the very least we don't
> want to be slowed down from the bio logic in the physically contigous
> pagecache flood case.
> 
> I just meant the bh isn't totally pointless and it could be shrunk as
> Arjan said in a private email.

bh represents a disk block.  It's a wrapper around a section of the
block device's pagecache pages.  We'll always need a representation
of disk blocks.  For filesystem metadata.

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