Date: Thu, 19 Apr 2012 22:46:22 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Andre Oppermann <andre@freebsd.org> Cc: current@freebsd.org, net@freebsd.org Subject: Re: Some performance measurements on the FreeBSD network stack Message-ID: <20120419204622.GA94904@onelab2.iet.unipi.it> In-Reply-To: <4F907011.9080602@freebsd.org> References: <20120419133018.GA91364@onelab2.iet.unipi.it> <4F907011.9080602@freebsd.org>
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On Thu, Apr 19, 2012 at 10:05:37PM +0200, Andre Oppermann wrote: > On 19.04.2012 15:30, Luigi Rizzo wrote: > >I have been running some performance tests on UDP sockets, > >using the netsend program in tools/tools/netrate/netsend > >and instrumenting the source code and the kernel do return in > >various points of the path. Here are some results which > >I hope you find interesting. > > Jumping over very interesting analysis... > > >- the next expensive operation, consuming another 100ns, > > is the mbuf allocation in m_uiotombuf(). Nevertheless, the allocator > > seems to scale decently at least with 4 cores. The copyin() is > > relatively inexpensive (not reported in the data below, but > > disabling it saves only 15-20ns for a short packet). > > > > I have not followed the details, but the allocator calls the zone > > allocator and there is at least one critical_enter()/critical_exit() > > pair, and the highly modular architecture invokes long chains of > > indirect function calls both on allocation and release. > > > > It might make sense to keep a small pool of mbufs attached to the > > socket buffer instead of going to the zone allocator. > > Or defer the actual encapsulation to the > > (*so->so_proto->pr_usrreqs->pru_send)() which is called inline, anyways. > > The UMA mbuf allocator is certainly not perfect but rather good. > It has a per-CPU cache of mbuf's that are very fast to allocate > from. Once it has used them it needs to refill from the global > pool which may happen from time to time and show up in the averages. indeed i was pleased to see no difference between 1 and 4 threads. This also suggests that the global pool is accessed very seldom, and for short times, otherwise you'd see the effect with 4 threads. What might be moderately expensive are the critical_enter()/critical_exit() calls around individual allocations. The allocation happens while the code has already an exclusive lock on so->snd_buf so a pool of fresh buffers could be attached there. But the other consideration is that one could defer the mbuf allocation to a later time when the packet is actually built (or anyways right before the thread returns). What i envision (and this would fit nicely with netmap) is the following: - have a (possibly readonly) template for the headers (MAC+IP+UDP) attached to the socket, built on demand, and cached and managed with similar invalidation rules as used by fastforward; - possibly extend the pru_send interface so one can pass down the uio instead of the mbuf; - make an opportunistic buffer allocation in some place downstream, where the code already has an x-lock on some resource (could be the snd_buf, the interface, ...) so the allocation comes for free. > >- another big bottleneck is the route lookup in ip_output() > > (between entries 51 and 56). Not only it eats another > > 100ns+ on an empty routing table, but it also > > causes huge contentions when multiple cores > > are involved. > > This is indeed a big problem. I'm working (rough edges remain) on > changing the routing table locking to an rmlock (read-mostly) which i was wondering, is there a way (and/or any advantage) to use the fastforward code to look up the route for locally sourced packets ? cheers luigi
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