On 11/12/12 10:23 PM, Peter Wemm wrote:
On Mon, Nov 12, 2012 at 10:11 PM, Alfred Perlstein <bri...@mu.org> wrote:
On 11/12/12 10:04 PM, Alfred Perlstein wrote:
On 11/12/12 10:48 AM, Alfred Perlstein wrote:
On 11/12/12 10:01 AM, Andre Oppermann wrote:
I've already added the tunable "kern.maxmbufmem" which is in pages.
That's probably not very convenient to work with. I can change it
to a percentage of phymem/kva. Would that make you happy?
It really makes sense to have the hash table be some relation to sockets
rather than buffers.
If you are hashing "foo-objects" you want the hash to be some relation to
the max amount of "foo-objects" you'll see, not backwards derived from the
number of "bar-objects" that "foo-objects" contain, right?
Because we are hashing the sockets, right? not clusters.
Maybe I'm wrong? I'm open to ideas.
Hey Andre, the following patch is what I was thinking
(uncompiled/untested), it basically rounds up the maxsockets to a power of 2
and replaces the default 512 tcb hashsize.
It might make sense to make the auto-tuning default to a minimum of 512.
There are a number of other hashes with static sizes that could make use
of this logic provided it's not upside-down.
Any thoughts on this?
Tune the tcp pcb hash based on maxsockets.
Be more forgiving of poorly chosen tunables by finding a closer power
of two rather than clamping down to 512.
Index: tcp_subr.c
===================================================================
Sorry, GUI mangled the patch... attaching a plain text version.
Wait, you want to replace a hash with a flat array? Why even bother
to call it a hash at that point?
If you are concerned about the space/time tradeoff I'm pretty happy with
making it 1/2, 1/4th, 1/8th the size of maxsockets. (smaller?)
Would that work better?
The reason I chose to make it equal to max sockets was a space/time
tradeoff, ideally a hash should have zero collisions and if a user has
enough memory for 250,000 sockets, then surely they have enough memory
for 256,000 pointers.
If you strongly disagree then I am fine with a more conservative
setting, just note that effectively the hash table will require 1/2 the
factor that we go smaller in additional traversals when we max out the
number of sockets. Meaning if the table is 1/4 the size of max sockets,
when we hit that many tcp connections I think we'll see an order of
average 2 linked list traversals to find a node. At 1/8, then that
number becomes 4.
I recall back in 2001 on a PII400 with a custom webserver I wrote having
a huge benefit by upping this to 2^14 or maybe even 2^16, I forget, but
suddenly my CPU went down a huge amount and I didn't have to worry about
a load balancer or other tricks.
-Alfred
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