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Re: scheduling question

On Jan 14, 2004, at 1:35 PM, Stefan Werner wrote:
I am writing a real-time application and am in the optimization phase right
now, playing with priorities and scheduling parameters. The critical thread
in my application is a loop around recv() that is waiting for UDP packages
and processes them upon reception. It should react on incoming packages as
quickly as possible and get the processing done in a certain time. Now I'm
wondering what's better suited for my thread: Real-time scheduling
(guaranteeing my thread a certain time slice) or a "regular" thread with an
insanely high priority? I tried some code snippets I found, one of them* is
setting my thread to THREAD_TIME_CONSTRAINT_POLICY, the other is setting it
to THREAD_PRECEDENCE_POLICY, in addition to setting
threadExtendedPolicy.timeshare = 0. I tried them both, and with each of them
my application performed much better than with standard threading. But
still, it could be a tad better, and I guess I should start tweaking the
parameters I pass to the functions. Before I spend too much time doing that,
I wonder what will give me better results: time constraint or precedence
policy? Will changing the process' priority improve anything or are my
threads scheduled independently of the process?

Handling network traffic in a realtime (as defined by THREAD_TIME_CONSTRAINT_POLICY) thread doesn't mix too well. Those threads are meant mostly for talking directly to external devices that can There are "helper" threads that process the data within the in-kernel network code somewhat isolating your thread from directly accessing the network hardware in that way. Your realtime thread may even interfere with the responsiveness of these helper threads in ways that might affect your own data. Even if you don't interfere with the processing of the in-kernel data, your response-time characteristics are going to be dominated by the scheduling behavior of these in-kernel threads. There are also situation where the network stack just borrows another user-level thread (that just happened to be in the network stack at the time) to drive completion of your packet handling (inbound and outbound). Obviously, you can't make these all realtime threads, so that too will affect your latency. For these reasons it is probably counter-productive to go all the way to a TIME_CONSTRAINT thread for this kind of data handling.

The THREAD_PRECEDENCE_POLICY isn't really a completely different policy from the normal timeshare (migrating priority) policy on the system. It's just a way to control a key aspect of that policy. In essence, you are telling the schedule that, within this one process, some threads should have precedence over others even if they have the same priority. Essentially, you are assigning ordinal values to the process' threads without having to know or guess an appropriate system-wide priority for the threads. The system doesn't guarantee to honor it, but will attempt to do so (fairness may dictate that we chose threads to execute out of ordinal order). However, the effect should be process-local, and therefore cannot be relied upon to have any system-wide effect whatsoever.

What has worked well for others in this type of situation is to use the THREAD_EXTENDED_POLICY notion to declare the thread as a "non-long-running" thread (by specifying the timeshare parameter as FALSE). As long as your thread is well behaved and gives up the CPU in reasonable amounts of time, it will always run at a fixed priority. As such, it will have significant preference over other threads in the system that are (or have been) consuming large amounts of CPU time (and are therefore likely to get in the time way of reasonable timeliness).

--Jim
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References: 
 >scheduling question (From: "Stefan Werner" <email@hidden>)



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