Hi folks,
one short question regarding net.ipv4.tcp_no_metrics_save sysctl
parameter. Code snippet is actually the following:
void tcp_update_metrics(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct dst_entry *dst = __sk_dst_get(sk);
if (sysctl_tcp_nometrics_save)
return;
Why this 'if' statement can't be moved before variables? I think fail
fast could save some instructions in this place. Any thoughts?
Here is the performance benchmarking results:
Test #1 (sysctl -w net.ipv4.tcp_no_metrics_save=0)
sum: 79726, avg: 1295ns
min: 453ns, max: 16487ns
Test #2 (sysctl -w net.ipv4.tcp_no_metrics_save=1)
sum: 77515, avg: 1181ns
min: 431ns, max: 15989ns
Benchmarked with Systemtap:
global c;
probe kernel.function("tcp_update_metrics").return
{
c <<< (gettimeofday_ns() - @entry(gettimeofday_ns()));
}
probe timer.s(60)
{
printf("sum: %d, avg: %dns\n", @count(c), @avg(c));
printf("min: %dns, max: %dns\n", @min(c), @max(c));
delete c;
exit();
}
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