Date: Mon, 12 Nov 2012 22:45:38 -0800 From: Alfred Perlstein <bright@mu.org> To: Peter Wemm <peter@wemm.org> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org> Subject: Re: auto tuning tcp Message-ID: <50A1EC92.9000507@mu.org> In-Reply-To: <CAGE5yCoj1dL9w-EMMi8iYMTOq9uUUHmFg4rMY7aPneUBHBv67Q@mail.gmail.com> References: <50A0A0EF.3020109@mu.org> <50A0A502.1030306@networx.ch> <50A0B8DA.9090409@mu.org> <50A0C0F4.8010706@networx.ch> <EB2C22B5-C18D-4AC2-8694-C5C0D96C07B3@mu.org> <50A13961.1030909@networx.ch> <50A14460.9020504@mu.org> <50A1E2E7.3090705@mu.org> <50A1E47C.1030208@mu.org> <CAGE5yCoj1dL9w-EMMi8iYMTOq9uUUHmFg4rMY7aPneUBHBv67Q@mail.gmail.com>
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On 11/12/12 10:23 PM, Peter Wemm wrote: > On Mon, Nov 12, 2012 at 10:11 PM, Alfred Perlstein <bright@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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