usleep

[Sharad Chandra]

 ,---- [Heiko Wundram (Beenic) wrote:]
| Am Montag, 25. Februar 2008 10:10:56 schrieb Sharad Chandra:
| > So does it mean, freebsd has limitation. sleeping will only work for its
| > value more than 1 milli sec because % of +- error value is comparitivly
| > low? I am curious to know, is there any method which sleeps for few
| > microseconds. Some one please give me link where to look for "select".
|
| No, this does not mean that FreeBSD has a limitation, but rather that
| FreeBSD is not a real-time operating system. You'll see the similar
| behaviour on pretty much any other operating system that is _not_ a RTOS
| (for example, Windows has a fixed resolution of non-Multimedia-Timers of
| around 10ms [where Multimedia Timers are a specific interface to sleep with
| a higher resolution, designed for Multimedia applications, but the
| resolution isn't that much higher, anyway], and on Linux, it also depends
| on the Tick-Frequency, similar to FreeBSD's HZ setting, but actually I
| don't know how the tickless Linux-kernel [which is implemented only in the
| latest versions, and not default anyway] behaves here).
|
| There is no way (AFAIK) for a userland application to _sleep_ for a finer
| grained amount of time than a timeslice, except in a busy loop which calls
| gettimeofday or similar repeatedly to break when the "timeout" occurs, but
| even then you don't have a guarantee that you won't sleep longer, simply
| because the process might be preempted just before the timeout occurs
| because another process also wants its timeslice.
|
| But, more specifically, why do you need to sleep for a short time in a user
| process anyway? Normally, you're blocking on some form of condition, which
| will "wake you up" as soon as the condition triggers and you can be
| scheduled (and in this "active scheduling" case the time-slice "rule"
| doesn't apply).

I got out-of-order captured packets on my starfire dual port card ie ack get 
lower time-stamp than its corresponding seq in tcp traffic. I don't have much 
nic card level knowledge, so decided to produce delay to send ack and 
increase it till no out-of-order packet. delay in 1000 micro seconds or above 
will not let me know the time difference figure. I am also searching for what 
kind of scheduling this card do in picking the one of buffer by time stamper 
and for what much time. Where should i look for these info?

|
| If you give some more info on what you're trying to do (and why you need
| such a high resolution sleep), maybe someone will be able to help you
| better and show you how to achieve what you're trying to do without
| blocking on a timeout.
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Thanks in advance.
Sharad Chandra