i have observed that using jumbo frames (mtu=9000) decreases the thruput
(i am timing one-way ttcp). trying w/ different mtu's i see 4096 give
me the best numbers:
1500 (default) ~846Mbps
4096 ~930Mbps <== highest
my setup is:
- 2 nodes connected directly (cross-over cable)
- each node: 2-way, 2.4G Xeon. 4G RAM., running RHEL3 (2.4.21-4.ELsmp)
- intel gige (82543GC), e1000 ver. (5.1.11-k1)
i think the cards are: 64bit/66Mhz PCI.
- ipv4.tcp_r/wmem and core.r/wmem_max set sufficiently high (512KB)
- using ttcp to xfer ~8GB one-way.
why doesn't my thruput increase with increase in MTU? is it because of
small number of rx/txdescriptors on 82543GC (max=256?) or something
are there any driver parameters that i can tune to get better numbers
with larger MTUs?