I was running some network tests taking samples every 10 seconds on an
nfs server connected to high-speed storage and a 10G NIC. The data was
read with collectl, which reads the data from /proc/net/dev. When I
look at the average packet sizes I come up with over 6K and jumbo frames
are not enabled. Naturally my first suspicion was collectl's math so I
went back to the original numbers and here's what I collected - the
'Net' at the beginning of each line is something I preface each line
with so I can tell which file it came from as I collect data from lots
of places. Anyhow, here are a couple of samples (don't know how they'll
show up after terminal wrapping):
Inter-| Receive
| Transmit
face |bytes packets errs drop fifo frame compressed
multicast|bytes packets errs drop fifo colls carrier compressed
Net eth2:1033991246323 770657768 0 885 0 0
0 0 209226011736 398954938 0 0 0 0 0 0
Net eth2:1034142397679 773022462 0 885 0 0
0 0 215709166026 399913993 0 0 0 0 0 0
Net eth2:1034294532107 775402505 0 886 0 0
0 0 222234389176 400880022 0 0 0 0 0 0
If I do the math for the first 2 samples it comes out as
215709166026-209226011736=6483154290 bytes and
399913993-398954938=959055 packets. The result of
6483154290/959055=6760 bytes/packet. And this is not just for a couple
of samples. Here's an example of collectl's output:
# Num Name InPck InErr OutPck OutErr Mult ICmp
OCmp IKB OKB
13:00:00 3 eth2: 236469 0 95905 0 0 0 0
14760 633120
13:00:10 3 eth2: 238004 0 96602 0 0 0 0
14856 637228
13:00:20 3 eth2: 238796 0 92460 0 0 0 0
14906 639296
13:00:29 3 eth2: 230974 0 93103 0 0 0 0
14419 618655
13:00:40 3 eth2: 234701 0 93414 0 0 0 0
14651 628492
13:00:50 3 eth2: 236121 0 94527 0 0 0 0
14739 632191
13:00:59 3 eth2: 237001 0 92259 0 0 0 0
14795 634433
note that these numbers as reported as KBs/sec and so have been divided
by 10 from the raw numbers shown in /proc/net/dev.
If I take a look at an earlier part of the same test when I'm doing
writes (the system sees them as InPck/IKB I see:
# Num Name InPck InErr OutPck OutErr Mult ICmp
OCmp IKB OKB
12:30:00 3 eth2: 268813 0 142581 0 0 0 0
383735 9100
12:30:10 3 eth2: 258240 0 136903 0 0 0 0
368605 8734
12:30:20 3 eth2: 283324 0 150269 0 0 0 0
404486 9587
12:30:30 3 eth2: 278165 0 147478 0 0 0 0
396937 9405
12:30:40 3 eth2: 280848 0 148938 0 0 0 0
400896 9506
and if I do the math on IKB (383735*1024/268813)=1462 which makes a
whole lot more sense. Any ideas as to what would cause the driver
incorrectly count the packets? I know the byte counts are correct
because this is an nfs server and the disk i/o rates are consistent
with the network rates.
-mark
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