topic Re: Async IO concept in Operating System - HP-UX

Async IO concept

Lalo Weng — Tue, 03 Apr 2001 03:01:51 GMT

Hi, All Master,

I'm reading Oracle Installation Guide on HP-UX, one term stops me.That is:Async IO. Could any one tell me more about it,I want to know its concept,background,principle, etc. Or Could you give me a link so that I will learn it from there.

Thanks in advance.

Re: Async IO concept

Philip Chan_1 — Tue, 03 Apr 2001 03:29:28 GMT

My interpretation of asynchronous I/O -> Async I/O means nowait I/O, or non-blocked I/O somebody called it. For example, in synchronous I/O mode if a process made a disk I/O request but the resouce isn't ready, the process will be blocked and wait, wait until the resource is available. This impose inefficiency on CPU usage. On the other hand, if the same process was re-written to adopt asynchronous I/O, the process WILL NOT WAIT for the I/O to be returned, it will made the request, then move on to perform the next other instructions, after finishing all queued instructions then the process will made a "select" system call to pick up its previously requested I/O then finish what it supposed to finish. This way CPU time will not be wasted on waiting (wio in the sar output).

My 2 cents.

~Philip

Re: Async IO concept

A. Clay Stephenson — Tue, 03 Apr 2001 12:45:18 GMT

Philip is correct and in almost all cases asynch io will give better performance but there is a downside. There is no assurance that the IO operation actually completes. You are trading speed for integrity. You will need to enable asynch_io in the kernel and rebuild the kernel.