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

[Goswin von Brederlow]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> 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().
>
> But it turns out that most of the filesystems we care about already use a 
> special case of "d_add()" that *already* replaces the dentry with another 
> one in some cases: "d_splice_alias()".
>
> So I bet that if we just taught "d_splice_alias()" to look at the inode, 
> and based on the inode just re-allocate the dentry to some other slab 
> cache, we'd already handle a lot of the cases!
>
> 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.
>
> 			Linus

You would only get it for dentries that live long (or your prediction
is awfully wrong) and then the reallocation amortizes over time if you
will. :)

MfG
        Goswin
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