On Fri, Jan 16, 2015 at 7:16 AM, Tom Marchant < [email protected]> wrote:
> On Thu, 15 Jan 2015 13:49:16 -0600, John McKown wrote: > > >5) SMT (hyperthreading) only on IFL and zIIP engines (not CPs). Apparently > >when running SMT, the individual threads can't match the speed of a > non-SMT > >CP, but their aggregate power may. > > I think of it this way. Remember in the old days when we ran > uniprocessors. Every time one job accessed DASD, it had to wait for the I/O > operation to complete. During that time, the operating system let another > job run. > > Today's processors have cache because main memory is _really_ slow > compared to the processor. When the processor accesses something at a > memory address, if the data at that location is in the cache, the processor > can access it in one clock cycle (if it is in the on-chip cache) or a few > clock cycles if it is farther away. > > If it is not in cache and the processor has to get the data from memory, > it takes hundreds of cycles. During that "wait" time, the processor will > switch to another thread to execute. Now that thread will run until it has > to access main memory, and the processor will switch again. > > In this way, at least some of the time that the processor is idle waiting > for data from memory, it is able to do useful work. > So, as an extremely silly example. SMT will not "help" in the following program: INSANE J INSANE I.e. in a "hard loop" which uses data & instructions which are "preloaded" into the i- and d- caches. Do both threads in a core share a single i- and d- cache? If something were dispatched on the other "thread", would it ever get any real CPU? Or is the z SMT like the Intel where both thread _could possibly_ actually run simultaneously in a single core? > > -- > Tom Marchant > > -- While a transcendent vocabulary is laudable, one must be eternally careful so that the calculated objective of communication does not become ensconced in obscurity. In other words, eschew obfuscation. 111,111,111 x 111,111,111 = 12,345,678,987,654,321 Maranatha! <>< John McKown ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
