University of Cambridge
On Thu, 8 Mar 2007, Dave Baukus wrote:
What's the word on netisr_direct ? Do people typically enable this feature ?
Direct dispatch is the default configuration for the network stack in FreeBSD
7.x. Many users have reported performance improvements, especially in high
packets-per-second receive environments. However, some users have reported
performance loss on some SMP systems with some workloads. Direct dispatch
will generally significantly lower the overhead of in-bound packet processing
in the network stack by avoiding context switches. It also introduces
parallelism in the in-bound network layer processing path by allowing
processing to occur in more than one thread at a time. However, you can see
reduced parallelism in some environments as the ithread no longer run
concurrently with the netisr. I would not use net.isr.direct in versions of
FreeBSD before 6.1, but it should be pretty safe for 6.1 and later.
Robert N M Watson
Computer Laboratory
University of Cambridge
net/netisr.c:
static int netisr_direct = 0;
SYSCTL_INT(_net_isr, OID_AUTO, direct, CTLFLAG_RW,
&netisr_direct, 0, "enable direct dispatch");
TUNABLE_INT("net.isr.direct", &netisr_direct);
/*
* Do direct dispatch only for MPSAFE netisrs (and
* only when enabled). Note that when a netisr is
* marked MPSAFE we permit multiple concurrent instances
* to run. We guarantee only the order in which
* packets are processed for each "dispatch point" in
* the system (i.e. call to netisr_dispatch or
* netisr_queue). This insures ordering of packets
* from an interface but does not guarantee ordering
* between multiple places in the system (e.g. IP
* dispatched from interfaces vs. IP queued from IPSec).
*/
if (netisr_direct && (ni->ni_flags & NETISR_MPSAFE)) {
isrstat.isrs_directed++;
/*
* NB: We used to drain the queue before handling
* the packet but now do not. Doing so here will
* not preserve ordering so instead we fallback to
* guaranteeing order only from dispatch points
* in the system (see above).
*/
ni->ni_handler(m);
--
Dave Baukus
[EMAIL PROTECTED]
972-479-2491
Fujitsu Network Communications
Richardson, Texas
USA
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