Re: Lost UDP packets
Re: Lost UDP packets
- Subject: Re: Lost UDP packets
- From: Jim Hunter <email@hidden>
- Date: Sun, 26 Sep 2004 20:13:47 -0500
> Date: Sat, 25 Sep 2004 14:23:24 -0700
> From: Justin Walker <email@hidden>
> Subject: Re: Lost UDP packets
> To: Mark Hunt <email@hidden>
> Cc: email@hidden
> Message-ID: <email@hidden>
> Content-Type: text/plain; charset=US-ASCII; format=flowed
>
>
> On Sep 25, 2004, at 13:43, Mark Hunt wrote:
>
>>Since for any given lost packet, another receiver gets it, I think I
>>can rule out collisions.
I agree.
>>I'm still waiting on a meaningful netstat from my beta-tester, but I'm
>>pretty sure the problem is in my receiving application. It's pretty
>>busy, so maybe my network thread (blocked in recvfrom) needs higher
>>priority?
Why are you ruling out either the NIC hardware or the driver being
the cause of the dropped packets? I don't see why the problem has to be
at the application level.
> Also, check stats on both sender and receiver(s). You haven't ruled
> out the sender yet, and at high packet generation rates, the CPU can
> easily overwhelm the driver send-queue limits (usually fairly small).
> For UDP, when this happens, the packets are dropped on the sending end
> (without error indications; only the counters are updated).
It can't be the sender, since the "lost" packets are actually being
received on some other nodes. If the transmitter's send queue was being
overrun, then the lost packets would not be received anywhere.
Are you using the built-in ethernet interface? I believe that it disables
link level flow control, so a burst of incoming packets could deplete
the NIC's supply of receive buffers and cause packets to be dropped.
Is your traffic rate "bursty", or relatively constant? If it is bursty,
you may be able to ride out the bursts either by enabling flow control
or by allocating more resources (such as the number of available receive
buffers), but both options would be driver dependant. If your packet rate
is relatively constant, and the sender is overrunning the receiver,
then there is nothing that you can do to prevent packets from being lost
eventually. This situation occurs with many network performance tests such
as nttcp or iperf when using UDP.
Jim.
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