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

[Jörn Engel <joern@logfs.org>]

On Sun, 16 September 2007 11:44:09 -0700, Linus Torvalds wrote:
> On Sun, 16 Sep 2007, Jörn Engel wrote:
> > 
> > My approach is to have one for mount points and ramfs/tmpfs/sysfs/etc.
> > which are pinned for their entire lifetime and another for regular
> > files/inodes.  One could take a three-way approach and have
> > always-pinned, often-pinned and rarely-pinned.
> > 
> > We won't get never-pinned that way.
> 
> That sounds pretty good. The problem, of course, is that most of the time, 
> the actual dentry allocation itself is done before you really know which 
> case the dentry will be in, and the natural place for actually giving the 
> dentry lifetime hint is *not* at "d_alloc()", but when we "instantiate" 
> it with d_add() or d_instantiate().
> 
> [...]
> 
> And yes, you'd end up with the reallocation overhead quite often, but at 
> least it would now happen only when filling in a dentry, not in the 
> (*much* more critical) cached lookup path.

There may be another approach.  We could create a never-pinned cache,
without trying hard to keep it full.  Instead of moving a hot dentry at
dput() time, we move a cold one from the end of lru.  And if the lru
list is short, we just chicken out.

Our definition of "short lru list" can either be based on a ratio of
pinned to unpinned dentries or on a metric of cache hits vs. cache
misses.  I tend to dislike the cache hit metric, because updatedb would
cause tons of misses and result in the same mess we have right now.

With this double cache, we have a source of slabs to cheaply reap under
memory pressure, but still have a performance advantage (memcpy beats
disk io by orders of magnitude).

Jörn

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