Re: min-write-size for a UDP socket to be POLLOUT cannot be set. (proposed fixes)

From: Ben Greear (
Date: Mon Dec 10 2001 - 16:14:47 EST

Richard B. Johnson wrote:

> On Mon, 10 Dec 2001, Ben Greear wrote:

>>I have 4M queue size. I have 4M-2k bytes already in the
>>queue (2k free). I have a 4k UDP buffer to write. I call
>>select and it says the socket is writable. However, in this
>>case I cannot actually write to the socket because I have only
>>2 of the 4k that I need... Now, I can detect the failure to send
>>and re-transmit, but that basically gets me into a tight loop because
>>select keeps saying I can write, and I keep trying. The tight loop
>>is doubly bad because the machine is already highly stressed or it's
>>buffers would never be so full....
>>I want select to only say I can write when I'm at XX (say, 64k) bytes of
>>free buffer-queue space...
> If you have a 4M queue size, it appears as though you are trying to
> use UDP where TCP should have been used. Normally, what you call the
> queue size, is set to contain you largest packet you will ever want to
> send. With this in mind, you don't even know if a fragmented packet
> can be routed if it's more than 64k in length so you would never try
> to send something larger than that under UDP.

Assume that I actually want to do what I said I did! :)

I never try to send
a packet bigger than 64k, the protocol doesn't handle it. But, I may try
to send 100000 60k UDP packets in very fast succession...which could fill up my
send queue.

Ben Greear <>       <Ben_Greear AT>
President of Candela Technologies Inc

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This archive was generated by hypermail 2b29 : Sat Dec 15 2001 - 21:00:18 EST