Re: [PATCH net-next] tcp: avoid expensive pskb_expand_head() calls

Rick Jones <rick.jones2@xxxxxx> · Wed, 18 Apr 2012 10:30:12 -0700

On 04/18/2012 10:16 AM, Eric Dumazet wrote:

On Wed, 2012-04-18 at 10:00 -0700, Rick Jones wrote:

Is the issue completely sent, or transmit completion processed?  I'd
think it is time to the latter that matters (and includes the former) yes?

I dont know. Fact is we process ACKs before clone skb is freed by TX
completion.

Does the ixgbe driver do transmit completions first when it gets a
receive interrupt, or is there still the chance that the receipt of the
last ACK for the 64KB skb will hit TCP before the driver has done the
free?  (Or does that not matter?)

It does transmit completions first, but that doesnt matter, since we
receive ACK before skb could be drained by NIC and returned to driver
for TX completion.

I was thinking more about the race if any between the ACK for the last 

byte of the 64 KB skb and the transmit completion processing freeing it 

in the driver.  But that may be moot.

Performance results on my Q6600 cpu and 82599EB 10-Gigabit card :
About 3% less cpu used for same workload (single netperf TCP_STREAM),
bounded by x4 PCI-e slots (4660 Mbits).

Three percent less or three percentage points less?  Including the
details of the netperf-reported service demand would make that clear.

netperf results are not precise enough, since my setup is limited by PCI
bandwidth. here are the "perf stat" ones

I'm confused -  Netperf's CPU utilization measurements (-c -C) and by 

extension service demand calculation should be able to see an overall 

three percentage point change in CPU util, even a three percent one.

Maybe someone can run the test on 20Gb/40Gb links, and NUMA machine.

Before patch :

# perf stat -r 5 -d -d -o RES.before taskset 1 netperf -H 192.168.99.1 -l 20

I'm still learning about perf, and the manpage I have for it does not 

discuss the -d option but is that doing system wide, or only in the 

context of the netperf process?

rick
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