question on select: How big can the empty buffer space be before select returns ready-to-write?

question on select: How big can the empty buffer space be before select returns ready-to-write?

[Ben Greear]

For instance, it appears that select will return that a socket is
writable when there is, say 8k of buffer space in it.  However, if
I'm sending 32k UDP packets, this still causes me to drop packets
due to a lack of resources...

Is there any IOCTL that can tell select how much space to require
before it thinks a socket is writable?

Many thanks,
Ben

-- 
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

Re: question on select: How big can the empty buffer space be before select returns ready-to-write?

[Chris Wright]

* Ben Greear (greearb@candelatech.com) wrote:
> For instance, it appears that select will return that a socket is
> writable when there is, say 8k of buffer space in it.  However, if
> I'm sending 32k UDP packets, this still causes me to drop packets
> due to a lack of resources...

udp has a fixed 8k max payload. did you try breaking up your packets?

cheers,
-chris

Re: question on select: How big can the empty buffer space be before select returns ready-to-write?

[Alan Cox]

> 
> * Ben Greear (greearb@candelatech.com) wrote:
> > For instance, it appears that select will return that a socket is
> > writable when there is, say 8k of buffer space in it.  However, if
> > I'm sending 32k UDP packets, this still causes me to drop packets
> > due to a lack of resources...
> 
> udp has a fixed 8k max payload. did you try breaking up your packets?

UDP has a 64K - headers max payload. 

Re: question on select: How big can the empty buffer space be before select returns ready-to-write?

[Ben Greear]

Alan Cox wrote:

>>* Ben Greear (greearb@candelatech.com) wrote:
>>
>>>For instance, it appears that select will return that a socket is
>>>writable when there is, say 8k of buffer space in it.  However, if
>>>I'm sending 32k UDP packets, this still causes me to drop packets
>>>due to a lack of resources...
>>>
>>udp has a fixed 8k max payload. did you try breaking up your packets?
>>
> 
> UDP has a 64K - headers max payload. 


Yes, and I am writing code to specifically try out large UDP
packet sizes, so limiting myself to a certain size is not at
all useful in this case....

Thanks,
Ben

-- 
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

Re: question on select: How big can the empty buffer space be before select returns ready-to-write?

[Christopher Friesen]

Chris Wright wrote:
> 
> * Ben Greear (greearb@candelatech.com) wrote:
> > For instance, it appears that select will return that a socket is
> > writable when there is, say 8k of buffer space in it.  However, if
> > I'm sending 32k UDP packets, this still causes me to drop packets
> > due to a lack of resources...
> 
> udp has a fixed 8k max payload. did you try breaking up your packets?

Are you sure about that? UDP has a 16-bit field for the length.  Thus the
standard technically allows for packet sizes (including header) of up to 2^16
(roughly 65K) bytes.

Chris

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Re: question on select: How big can the empty buffer space be before select returns ready-to-write?

[Chris Wright]

* Christopher Friesen (cfriesen@nortelnetworks.com) wrote:
> Chris Wright wrote:
> > 
> > * Ben Greear (greearb@candelatech.com) wrote:
> > > For instance, it appears that select will return that a socket is
> > > writable when there is, say 8k of buffer space in it.  However, if
> > > I'm sending 32k UDP packets, this still causes me to drop packets
> > > due to a lack of resources...
> > 
> > udp has a fixed 8k max payload. did you try breaking up your packets?
> 
> Are you sure about that? UDP has a 16-bit field for the length.  Thus the
> standard technically allows for packet sizes (including header) of up to 2^16
> (roughly 65K) bytes.

no, you are absolutely right, it's 16 bits.  sorry for spewing
misinformation.

cheers,
-chris