On Thu, 27 Sep 2007 11:58:01 +0800 "Majumder, Rajib" <[EMAIL PROTECTED]> wrote:
> Hi, > > We have observed 40ms latency spikes in TCP connections in "burst" type of > traffic. This affects regular TCP sockets. We observed this issue in kernels > of 2.4.21 and kernel 2.6.5. Unfortunately, 2.6.5 is out of my short term memory at this point. I do remember that 2.6.5 used BIC for congestion control, and there were some math errors in the congestion control logic that caused it to be way to aggressive. > > Aparently, this seems to be fixed in 2.6.19. > > Can someone throw some light on this? My guess is that the addition of the SACK hinting might be the major win. The code takes 3 passes over the SACK list, so with large outstanding data that was a major bottleneck, not sure if it was 4ms worth though. > > Is this a congestion control/avoidance issue? What congestion control > algorithm is used before 2.6.8? Default congestion control in early 2.6 was BIC, then after CUBIC stabilized it was made the default in 2.6.19. Another thing that may cause changes in latency is Appropriate Byte Counting (ABC). It was added in 2.6.14, but then turned off by default in 2.6.18. The problem is that ABC caused performance problems with some applications that sent messages as many small writes. -- Stephen Hemminger <[EMAIL PROTECTED]> - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
