Hi Eric, On Tue, May 26, 2015 at 09:23:55AM -0700, Eric Dumazet wrote: > On Tue, 2015-05-26 at 10:25 -0400, Ido Yariv wrote: > > The Tail Loss Probe RFC specifies that the PTO value should be set to > > max(2 * SRTT, 10ms), where SRTT is the smoothed round-trip time. > > > > The PTO value is converted to jiffies, so the timer might expire > > prematurely. This is especially problematic on systems in which HZ=100. > > > > To work around this issue, increase the number of jiffies by one, > > ensuring that the timeout won't expire in less than 10ms. > > > > Signed-off-by: Ido Yariv <idox.ya...@intel.com> > > --- > > net/ipv4/tcp_output.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > > index 534e5fd..6f57d3d 100644 > > --- a/net/ipv4/tcp_output.c > > +++ b/net/ipv4/tcp_output.c > > @@ -2207,7 +2207,7 @@ bool tcp_schedule_loss_probe(struct sock *sk) > > if (tp->packets_out == 1) > > timeout = max_t(u32, timeout, > > (rtt + (rtt >> 1) + TCP_DELACK_MAX)); > > - timeout = max_t(u32, timeout, msecs_to_jiffies(10)); > > + timeout = max_t(u32, timeout, msecs_to_jiffies(10) + 1); > > > > /* If RTO is shorter, just schedule TLP in its place. */ > > tlp_time_stamp = tcp_time_stamp + timeout; > > Have you really hit an issue, or did you send this patch after all these > msecs_to_jiffies() discussions on lkml/netdev ?
This actually fixed a specific issue I ran into. This issue caused a degradation in throughput in a benchmark which sent relatively small chunks of data (100KB) in a loop. The impact was quite substantial - with this patch, throughput increased by up to 50% on the platform this was tested on. > > Not sure this is the right fix. > > TLP was really tested with an effective min delay of 10ms. > > Adding 10% for the sake of crazy HZ=100 builds seems a high price. > (All recent TCP changes were tested with HZ=1000 BTW ...) > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > index > 534e5fdb04c11152bae36f47a786e8b10b823cd3..5321df89af9b59c6727395c489e6f9b2770dcd5e > 100644 > --- a/net/ipv4/tcp_output.c > +++ b/net/ipv4/tcp_output.c > @@ -2208,6 +2208,9 @@ bool tcp_schedule_loss_probe(struct sock *sk) > timeout = max_t(u32, timeout, > (rtt + (rtt >> 1) + TCP_DELACK_MAX)); > timeout = max_t(u32, timeout, msecs_to_jiffies(10)); > +#if HZ <= 100 > + timeout = max_t(u32, timeout, 2); > +#endif > > /* If RTO is shorter, just schedule TLP in its place. */ > tlp_time_stamp = tcp_time_stamp + timeout; This was actually the first incarnation of this patch. However, while the impact of this issue when HZ=100 is the greatest, it can also impact other settings as well. For instance, if HZ=250, the timer could expire after a bit over 8ms instead of 10ms, and 9ms for HZ=1000. By increasing the number of jiffies, we ensure that we'll wait at least 10ms but never less than that, so for HZ=1000, it'll be anywhere between 10ms and 11ms instead of 9ms and 10ms. Thanks, Ido. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
