Re: [patch][rfc] ddds: "dynamic dynamic data structure" algorithm, for adaptive dcache hash table sizing (resend)

Christoph Hellwig
2008-10-07T07:18:27Z

On Tue, Oct 07, 2008 at 09:02:25AM +0200, Nick Piggin wrote:
> (resending with correct netdev address)
>
> Hi,
>
> I thought I should quickly bring this patch up to date and write it up
> properly, because IMO it is still useful. I earlier had tried to turn the
> algorithm into a library that could be plugged into with specific lookup
> functions and such, but that got really nasty and also difficult to retain
> a really light fastpath. I don't think it is too nasty to open-code it...
>
> Describe the "Dynamic dynamic data structure" (DDDS) algorithm, and implement
> adaptive dcache hash table sizing using DDDS.
>
> The dcache hash size is increased to the next power of 2 if the number
> of dentries exceeds the current size of the dcache hash table. It is decreased
> in size if it is currently more than 3 times the number of dentries.
>
> This might be a dumb thing to do. It also currently performs the hash resizing
> check for each dentry insertion/deletion, and calls the resizing in-line from
> there: that's bad, because resizing takes several RCU grace periods. Rather it
> should kick off a thread to do the resizing, or even have a background worker
> thread checking the sizes periodically and resizing if required.
>
> With this algorithm, I can fit a whole kernel source and git tree in my dcache
> hash table that is still 1/8th the size it would be before the patch.
>
> I'm cc'ing netdev because Dave did express some interest in using this for
> some networking hashes, and network guys in general are pretty cluey when it
> comes to hashes and such ;)

Without even looking at the code I'd say geeting the dcache lookup data
structure as a hash is the main problem here. Dcache lookup is
fundamentally a tree lookup, with some very nice domain splits
(superblocks or directories). Mapping these back to a global hash is
a rather bad idea, not just for scalability purposes.