On 23.09.2012 17:35, Andrey Zonov wrote:
On 9/20/12 11:35 AM, Eggert, Lars wrote:
Hi,
On Sep 20, 2012, at 9:25, Andrey Zonov <[email protected]> wrote:
Some of them may be read google's article about tuning TCP parameters
[1]. I convert most of TCP timers to sysctls [2] and we are using this
patch for few months. We tuned net.inet.tcp.rtobase and
net.inet.tcp.syncache.rexmttime and it gives good results (especially in
conjunction with cc_htcp(4)).
can you share some measurements that quantify the results?
When we set net.inet.tcp.syncache.rexmttime=200 and
net.inet.tcp.syncache.rexmtlimit=7 for our external web service, the
number of duplicated SYN was reduced in four times.
This isn't surprising. You're simply trading retransmits by the client
with retransmits by the server. Whether this is within the overall packet
conservation principle is not clear. On the timeline it may be an advantage.
I'm not comfortable with the rather low retransmit time you've chosen
here. Considering higher RTT's (e.g. Hawaii or JP/CN) and the bufferbloat
problem this may be too low. When it is to be tuned, then something in the
range of 500-1000ms may be more realistic to avoid spurious retransmits.
When a SYN or SYN/ACK retransmit happens, the initial CWND should be reduced
per the applicable RFC's as this indicates packet loss on the downstream.
--
Andre
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