Messages in this thread | | | Date | Tue, 7 Oct 2008 03:18:27 -0400 | From | Christoph Hellwig <> | Subject | Re: [patch][rfc] ddds: "dynamic dynamic data structure" algorithm, for adaptive dcache hash table sizing (resend) |
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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.
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