On Mon, 2016-03-28 at 09:15 -0700, Rick Jones wrote:
> On 03/25/2016 03:29 PM, Eric Dumazet wrote:
> > UDP sockets are not short lived in the high usage case, so the added
> > cost of call_rcu() should not be a concern.
>
> Even a busy DNS resolver?
If you mean that a busy DNS resolver spends _most_ of its time doing :
fd = socket()
bind(fd port=0)
< send and receive one frame >
close(fd)
(If this is the case, may I suggest doing something different, and use
some kind of caches ? It will be way faster.)
Then the result for 10,000,000 loops of <socket()+bind()+close()> are
Before patch :
real 0m13.665s
user 0m0.548s
sys 0m12.372s
After patch :
real 0m20.599s
user 0m0.465s
sys 0m17.965s
So the worst overhead is 700 ns
This is roughly the cost for bringing 960 bytes from memory, or 15 cache
lines (on x86_64)
# grep UDP /proc/slabinfo
UDPLITEv6 0 0 1088 7 2 : tunables 24 12 8 :
slabdata 0 0 0
UDPv6 24 49 1088 7 2 : tunables 24 12 8 :
slabdata 7 7 0
UDP-Lite 0 0 960 4 1 : tunables 54 27 8 :
slabdata 0 0 0
UDP 30 36 960 4 1 : tunables 54 27 8 :
slabdata 9 9 2
In reality, chances that UDP sockets are re-opened right after being
freed and their 15 cache lines are very hot in cpu caches is quite
small, so I would not worry at all about this rather stupid benchmark.
int main(int argc, char *argv[]) {
struct sockaddr_in addr;
int i, fd, loops = 10000000;
for (i = 0; i < loops; i++) {
fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd == -1) {
perror("socket");
break;
}
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
if (bind(fd, (const struct sockaddr *)&addr, sizeof(addr)) ==
-1) {
perror("bind");
break;
}
close(fd);
}
return 0;
}
