Re: Linux 2.2.2 TCP delays every 41st small packet by 10-20 ms

Josip Loncaric (josip@icase.edu)
Fri, 26 Feb 1999 15:12:12 -0500


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Andrea Arcangeli wrote:
>
> On Thu, 25 Feb 1999, Josip Loncaric wrote:
>
> >The conclusion is this: in Linux TCP, every N-th small packet is delayed
> >by 1-2 "jiffies" (defined by the 100 Hz system clock). For Linux kernel
>
> This 1-2 jiffies made me thing again at the PSH thing. Probably I am
> completly out but could you try this patch?

Your patch improved things by a factor of 20. The easiest way for me to
explain how is to enclose a GIF file (<7KB, I hope this is not too
much). This plots intermessage time in clock ticks (400 MHz clock),
sorted from largest to smallest. Red dots are the unmodified Linux
kernel 2.2.2; green dots are after Andrea's patch. Horizontal scales
(message number after sorting by delay) are not directly comparable
because a lot more messages could be sent after Andrea's patch, so the
green curve is shifted to the right in this log-log plot.

Atypically large delays now cluster around 1 millisecond instead of 20
milliseconds. In both cases, a vast majority of the messages are spaced
by less than 100 microseconds. Even after the patch, five delays of 200
milliseconds out of the 420,602 measured were seen, but this might be
due to some unavoidable system activity during the test.

Sincerely,
Josip

-- 
Dr. Josip Loncaric, Senior Staff Scientist        mailto:josip@icase.edu
ICASE, Mail Stop 403                        http://www.icase.edu/~josip/
NASA Langley Research Center             mailto:j.loncaric@larc.nasa.gov
Hampton, VA 23681-2199, USA    Tel. +1 757 864-2192  Fax +1 757 864-6134
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