Some performance measurements on the FreeBSD network stack
andre at freebsd.org
Thu Apr 19 21:19:11 UTC 2012
On 19.04.2012 22:46, Luigi Rizzo wrote:
> 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.
Robert did the per-CPU mbuf allocator pools a few years ago.
> What might be moderately expensive are the critical_enter()/critical_exit()
> calls around individual allocations.
Can't get away from those as a thread must not migrate away
when manipulating the per-CPU mbuf pool.
> The allocation happens while the code has already an exclusive
> lock on so->snd_buf so a pool of fresh buffers could be attached
Ah, there it is not necessary to hold the snd_buf lock while
doing the allocate+copyin. With soreceive_stream() (which is
experimental not enabled by default) I did just that for the
receive path. It's quite a significant gain there.
IMHO better resolve the locking order than to juggle yet another
> 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;
That would require to cross-pointer the rtentry and whatnot again.
We want to get away from that to untangle the (locking) mess that
eventually results from it.
> - 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 ?
No. The main advantage/difference of fastforward is the short code
path and processing to completion.
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