Re: 2 or 4 cpu's running Informix OnLine 7.xx

David L. Kosenko wrote: > > Mike Segel wrote: > > With SMP, neither cpu is *idle*. They both or all, pull procs from the > > same > > stack. > > Yes, but if you only have one CPU VP and nothing else really running on the > machine, the second cpu will be idle a lot of the time. Two cpus cannot run > the same process simultaneously. > Same Process true. But, when is there only one process on the stac? Hmmm... Let me think. This is UNIX right? How many system procs are running? Lets take a quick look : inetd init nfsd gettys syncer syslog (OK, I think I made my point.) > Other's have pointed out situations where a 2-proc is best tuned as a uni, such as when > your applications run on the same machine as the server. Also consider those platforms > where kernel async i/o is not supported. If you are doing a lot of i/o in your app, the > aio vps are going to be very active, and could force the cpu vps to get context switched > more than is ideal. In those situations, it may be worthwhile to tune with the single > cpu vp and leave the other physical processor for this other server work. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This type of thinking is flawed. You can't control which CPU gets what in an SMP machine. Some OSs allow you to limit a task or set of tasks to one CPU. But they do not limit other processes to NOT use that CPU. (IBM AIX 4.1.3 does this.) Look, context switching occurs naturally in all multitasking OSes. Yes virgina there is *overhead* in a context switch. (Lets define Overhead is time spent doing a systems management task within the kernel.) Because the amount of *overhead* is not a linear progression, adding cpus will not yield a linear performance gain. Tasks switch easily. Threads do not. Since a thread shares a memory segment with its parent, it will be blocked when it tries to run on a different CPU and it's parent is running. (mutex lock) (And vice a vera.)[Note: This is all theoretical. Different implementations of operating systems will vary.] Context switching will occur for one of three reasons: 1) The time slice is up. 2) Blocked resource, or waiting for a resource like I/O. 3) Pre-empted by the kernel. (Deadlock detection/advoidence, watchdog alert, etc ...) [I'm sure there is something else but hey, what do I know?] BTW, are aio vps threads or tasks. If threads, then how does Informix handle mutex between aio vps? Depending on how the OS is written, and how the software is written, performace and tuning will vary. There are no set in stone rules. Its still a gut and intuitive process. My point was that individuals were thinking in terms of each physical cpus to be separate on an SMP box. The point of SMP is that they should be thinking in terms of a single cpu. Now clustering on the other hand .... [Then you have a whole different ball game and process migration means alot! ;-] > -- > Dave Kosenko, Informix Professional Services Mikey, Some guy found off the street! #include <std_disclaimer.h>