From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48EB11BB.2060704@cosmosbay.com> Date: Tue, 07 Oct 2008 09:37:31 +0200 From: Eric Dumazet MIME-Version: 1.0 Subject: Re: [patch][rfc] ddds: "dynamic dynamic data structure" algorithm, for adaptive dcache hash table sizing (resend) References: <20081007064834.GA5959@wotan.suse.de> <20081007070225.GB5959@wotan.suse.de> In-Reply-To: <20081007070225.GB5959@wotan.suse.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Linux Kernel Mailing List , Linux Memory Management List , netdev@vger.kernel.org, Paul McKenney List-ID: Nick Piggin a ecrit : > (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 ;) > Thanks for reminding us this interesting stuff. And yes, IP route cache could use same algo. That is particularly interesting because it has a /proc/net/rt_cache accessor that needs to sequentially scan this hash table (potentialy with many empty slots), while dcache doesnt have such killer. > > +static struct dcache_hash *alloc_dhash(int size) > +{ > + struct dcache_hash *dh; > + unsigned long bytes; > + unsigned int shift; > + int i; > + > + shift = ilog2(size); > + BUG_ON(size != 1UL << shift); > + bytes = size * sizeof(struct hlist_head *); > + > + dh = kmalloc(sizeof(struct dcache_hash), GFP_KERNEL); > + if (!dh) > + return NULL; > + > + if (bytes <= PAGE_SIZE) { > + dh->table = kmalloc(bytes, GFP_KERNEL); > + } else { > + dh->table = vmalloc(bytes); > + } Here we probably want to use a hashdist/NUMA enabled vmalloc(). That is, regardless of current numa policy of *this* thread, we want to spread hash table on all nodes. Also, struct dcache_hash being very small, you want to force it to use an exclusive cache line, to be sure it wont share it with some higly modified data... struct dcache_hash { struct hlist_head *table; unsigned int shift; unsigned int mask; } __cacheline_aligned_in_smp; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org