4. Performance
As described previously the performance of the Hercules Emulator depends heavily on the underlying PC
hardware. It is therefore not possible to give exact performance specifications of any particular operating
system running under Hercules. Some practical values from user experiences across several machines
presented here provide some guidelines though.
The performance of Hercules is measured in two values, MIPS and I/O rate. Both of these values are pre-
sented on the Hercules console and are refreshed every second independently of the PANRATE control
statement. See Hercules User Reference for details on the PANRATE statement.
4.1 MIPS
MIPS is an abbreviation of “Million Instructions Per Second” and presents a measure of the number of
instructions the CPU is executing in one second. MIPS is a measure of a computer's processor speed.
However this measure is useful only among processors with the same instruction set as different instruc-
tion sets take different numbers of instructions to do the same job. Many of the reported MIPS values
represent 'Peak' execution rates on artificial instruction sequences with few branches, whereas realistic
workloads consist of a mix of instructions some of which take longer to execute than others.
The performance of the memory hierarchy greatly affects processor performance, an issue also not con-
sidered in simple MIPS comparisons. In an attempt to address these issues researchers have created
standardized tests such as "SpecInt" to measure the real effective performance in commonly used
applications. The use of raw MIPS as a measure of overall system performance has fallen into disuse.
MIPS is sometimes pejoratively referred to as "Meaningless Indicator of Processor Speed" or "Meaning-
less Information Provided by Salespeople".
The Hercules console reports the MIPS rate for the emulated S/370, ESA/390 or z/Architecture instruc-
tions, not the underlying executed instructions of the hardware. As implied previously the MIPS rate can
vary significantly depending on the executed instruction and whether the instructions can be processed
entirely in cache, as can happen in a tight loop.
It is difficult to determine how the speed of the Hercules emulation corresponds to a real mainframe. This
is due to difficulties in comparing real mainframe hardware to PCs (or servers) as well as the actual per-
formance measurement itself. Hercules shows its processing speed in MIPS. Compared to the earlier
IBM System/360 and System/370 hardware, it is safe to say that Hercules will outperform them when it is
running on moderately powerful hardware, whereas newer IBM System/390 and z/Series hardware still is
much faster than the emulation.
On a Celeron 300 you should see an execution speed of 1 to 2 MIPS, which is enough to run OS/360
(MFT or MVT) or MVS 3.8 with a response time better than that of a 3033 from the 1970's. It's also fast
enough to run VSE/ESA with an acceptable response time. On a more recent system with a 2 GHz
Pentium processor, you may see the system peak at around 30 MIPS which is enough to run Linux/390
or z/OS with a light workload.
Performance on server class machines is now fairly respectable. For example, on a dual-core Intel Xeon
with "Hyperthreading" (4 CPUs) running at 3.46 GHz, you might expect to see a sustained MIPS rate of
40 to 60 MIPS. A dual-processor quad-core Mac Pro (8 cores, 3 GHz) will sustain over 150 MIPS. For
anyone who is prepared to spend a considerable amount of money on their Hercules system, there are
reports that a sustained 300+ MIPS rate has been achieved on an Intel Core i7 processor running at 3.75
GHz using all four cores plus "Hyperthreading" (8 CPUs).
The speed however depends greatly on the executed instructions. Instructions that are very expensive to
emulate can even on fast systems still be around 1 - 2 MIPS, see the tables further below for details.
Hercules Emulator - Installation Guide Page 16