Re: sched_yield() makes OpenLDAP slow

From: Robert Hancock
Date: Fri Aug 19 2005 - 22:33:40 EST


Howard Chu wrote:
You assume that spinlocks are the only reason a developer may want to yield the processor. This assumption is unfounded. Case in point - the primary backend in OpenLDAP uses a transactional database with page-level locking of its data structures to provide high levels of concurrency. It is the nature of such a system to encounter deadlocks over the normal course of operations. When a deadlock is detected, some thread must be chosen (by one of a variety of algorithms) to abort its transaction, in order to allow other operations to proceed to completion. In this situation, the chosen thread must get control of the CPU long enough to clean itself up, and then it must yield the CPU in order to allow any other competing threads to complete their transaction. The thread with the aborted transaction relinquishes all of its locks and then waits to get another shot at the CPU to try everything over again. Again, this is all fundamental to the nature of transactional programming. If the 2.6 kernel makes this programming model unreasonably slow, then quite simply this kernel is not viable as a database platform.

I fail to see how sched_yield is going to be very helpful in this situation. Since that call can sleep from a range of time ranging from zero to a long time, it's going to give unpredictable results.

It seems to me that this sort of thing is why we have POSIX pthread synchronization primitives.. sched_yield is basically there for a process to indicate that "what I'm doing doesn't matter much, let other stuff run". Any other use of it generally constitutes some kind of hack.

--
Robert Hancock Saskatoon, SK, Canada
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Home Page: http://www.roberthancock.com/

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