Date: Wed, 09 May 2001 12:54:03 +0200 From: Oscar-Ivan Lepe-Aldama <oscar@ac.upc.es> To: net@freebsd.org Subject: On the performance of the xl driver Message-ID: <3AF921CA.5C446E40@ac.upc.es>
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Hi,
I have a 4.1.1 box set up as a router that seems to have trouble dealing
with packet rates exceeding 1000 pkt/s. The router is a 100Mhz Pentium
with 16MB of RAM and two 3Com905B-TX ethernet adapters attached each to
a Ethernet subnetwork. The problem manifests itself when some packets
experience a huge forwarding latency, relatively to the average value.
The average forwarding latency time is 52us while the troubled packets
exhibit a latency of up to 6000us. I have observed that this is due to a
call to the xl_stats_update() routine while the kernel is at a high
priority level (perhaps splimp). This happens as part of the xl_intr
routine, when the network-adapter's interrupt status word has its
XL_STAT_STATSOFLOW bit set. The xl_stats_update() takes almost 6000us to
complete and processing for whichever packet that arrives during this
time will be deferred. Moreover, I have observed that this only happens
when the packet rate is 1000 pkt/s or more. At lower rates, the
XL_STAT_STATSOFLOW bit is never set because the network-adapter's
statistics buffer is periodicaly clean at a "sufficient" pace -the
xl_stats_update() is called from softclock() at a low kernel's priority
level.- To make matters worst, this is a phenomenon directly
proportional to the packet rate.
So the question is weather this is a bug or not and why.
Another point. To temporarly solve this I have disable the network
adapter's XL_STAT_STATSOFLOW interrupt (at if_xlreg.h, #define XL_INTRS)
but I don't know what are the implications of this, so any comments are
welcome.
Best regards,
--
========================================================================
0 0 0 Oscar-Ivan Lepe-Aldama | UPC-Campus Nord, DAC
0 0 0 e-mail: oscar@ac.upc.es | Modul D6, despatx 116
0 0 0 phone: +34 93 401 7187 | Jordi Girona, 1-3
U P C fax: +34 93 401 7055 | 08034 Barcelona - SPAIN
WWW: http://www.ac.upc.es/homes/oscar/
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