Re: multithread in INFORMIX, how?

Mike Segel wrote: > Chao Y. Din wrote: > > Saeed Talebbeik wrote:[Discussion about whether it is better on a 2 processor machine to tune DSA as an SMP or as a uniproc.] > Someone with no or little understanding of how SMP works, or > how to tune an engine started this rumor. Actually, people with a lot of experience with OnLine and SMP machines originated this advice. It is based in a fair amount of testing. > Please remember, that with SMP, no application knows which physical CPU > it will run on, and how many cpus physically exist. > > Before anyone starts in, yes, with some operating system dependent > features, you can bind a process to a CPU, and yes, you can set a > parameter telling Informix how many CPU's you have. But, these do not > contridict my earlier statements. > > Please just think about it..... You also need to consider that DSA tuned with Multiprocessor OFF elimates a large number of semaphore operations (as they are no longer necessary with a single CPUVP). In a significant number of situations, this results in a net gain in performance over a 2 CPUVP-tuned system. Depending on the nature of the use of the server, it may be beneficial to tune this way. Also consider the case where the application and the server run on the same machine. Since DSA is designed to "hog" a cpu (it will attempt to keep itself in a run state as much as possible), tuning 2 VPs causes the application to vie with the server for cpu cycles. If you have enough applications running, the amount of context switching necessary can result on poorer overall performance than sticking with one cpu (which the server will monopolize) and allowing the other for applications. While this is a simplified example (not taking into account how the various processes are prioritized and how time slicing is done on a particular o/s), the general idea is there. The thing to bear in mind is not to ELIMINATE FROM CONSIDERATION tuning Online with a single cpu vp on a 2 processor system. Try it: if the performance is worse, go back to 2 VPS (and, in fact, in some cases we have found performance advantages to tuning MORE cpuvps that physical cpus); if it is better and to your satisfaction, stick with it. There are more things in heaven and earth, Mr. Segel, than are dreamt of in your philosophy... -- Dave Kosenko, Informix Professional Services *********************************************** "I look back with some satisfaction on what an idiot I was when I was 25, but when I do that, I'm assuming I'm no longer an idiot." - Andy Rooney