select() doesn't respect SO_RCVLOWAT ?

Felix Matathias
2005-03-10T22:11:35Z

Dear all,

I am running a 2.4.21-9.0.3.ELsmp #1 kernel and I can setsockopt and
getsockopt correctly the SO_RCVLOWAT option, but select() seems to mark a
socket readable even if a single byte is ready to be read. Then, a read()
blocks until the specified number of bytes in SO_RCVLOWAT makes it to the
socket buffer.

This is the exact opposite behaviour of what I yould have
expected/desired. Our application receives data at many Khz rate and we
want to avoid reading the socket until a predetermined amount of data is
sent, to avoid partial reads. SO_RCVLOWAT seemed to be a nice way to
implement that.

An earlier message by Alan Cox was a bit cryptic:

"But is the cost of all those special case checks and all the handling
for it such as select computing if enough tcp packets together accumulated
worth the cost on every app not using LOWAT for the microscopic gain given
that essentially nobody uses it."

Does this mean that select() in Linux will wake up no matter what
SO_RCVLOWAT is set to ?

Best Regards,
Felix Matathias

P.S. I would appreciate if you could also cc your response to me.

--

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Felix Matathias of Columbia University, Nevis Labs

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