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

Eric Dumazet
2008-10-07T07:37:31Z

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;