Re: problems achieving decent throughput with latency.

[Ben Greear <greearb@candelatech.com>]

John Bradford wrote:
>>>TCP can only send into a pipe as fast as it can see the
>>>ACKs coming back.  That is how TCP clocks its sending rate,
>>>and latency thus affects that.
>>
>>Wouldn't you just need larger windows?  The problem is latency, not 
>>bandwidth.
> 
> 
> Exactly - the original post says that no problems are experienced
> using UDP, which backs that up.

I started poking around, and found the tcp_mem, tcp_rmem, and tcp_wmem
tunables in /proc/sys/net/ipv4...

If I change the values, I see up to 25Mbps with 25ms of latency.
It would go higher, but I have uncovered a performance bug in my code that
drops a packet every now and then at those higher rates, so that backs tcp
off quickly.  I should have that fixed this evening and will continue testing.


Here are the values that I used.  The documentation I found is not overly
descriptive, so if anyone has any suggestions for improving my tunings, please
let me know!

Also, if it's as simple as allocating a few more buffers for tcp, maybe we
should consider defaulting to higher in the normal kernel?  (I'm not suggesting
**my** numbers..)

# See the kernel documentation: Documentation/networking/ip-sysctl.txt
my $tcp_rmem_min     = 4096;
my $tcp_rmem_default = 256000;  # TCP specific receive memory pool size.
my $tcp_rmem_max     = 3000000;  # TCP specific receive memory pool size.

my $tcp_wmem_min     = 4096;
my $tcp_wmem_default = 256000;  # TCP specific receive memory pool size.
my $tcp_wmem_max     = 3000000;  # TCP specific receive memory pool size.

my $tcp_mem_lo       = 20000000; # Below here there is no memory pressure.
my $tcp_mem_pressure = 30000000; # Can use up to 30MB for TCP buffers.
my $tcp_mem_high     = 30000000; # Can use up to 30MB for TCP buffers.




> 
> John.
> -
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-- 
Ben Greear <greearb@candelatech.com>       <Ben_Greear AT excite.com>
President of Candela Technologies Inc      http://www.candelatech.com
ScryMUD:  http://scry.wanfear.com     http://scry.wanfear.com/~greear