The sections below deal with the x86 server components which have an effect on the CPU performance.
CPUs
The CPUs available are split logically into CPUs for applications under BS2000 (BS2000 CPUs), and CPUs on which BS2000 I/Os and administration tasks for the x86 server are handled by X2000 (I/O processors).
Depending on the model, up to 16 CPUs can be used as BS2000 CPUs. 25% of the available CPUs are used as I/O processors.
A special X2000 firmware layer supports execution of the BS2000 operating system and the compatible execution of /390 applications on the BS2000 CPUs, and also implementation of input/output operations on the I/O processors.
CISC firmware, JIT390
The CISC firmware (CISCFW) is the firmware component for mapping nonprivileged
/390 code to x86-64 code. It complements x86-64 mode and allows existing /390 code to be run on an object-compatible basis (synonym in /390 mode: compatibility mode) on x86-64 hardware.
The CISC firmware contains the component JIT390, a Just-In-Time /390 code compiler which converts the /390 code to x86-64 code at execution time. A code block is compiled only when it is executed and stored in a task-local JIT buffer. When the code block is executed again the code which has already been compiled and optimized is executed directly from the JIT buffer. Reusing predefined and optimized code sections is considerably faster than renewed interpretation and at the same time places less of a load on the processor. The JIT buffer is created as resident memory for the purpose of further optimization.
Existing (nonprivileged) customer applications still run on an object-compatible basis in /390 code using the CISC firmware. This applies both for applications which were generated with ASSEMBH and for programs generated with the BS2000 compilers for higher-level programming languages.
The effective performance of a server with x86 architecture depends on the number of system calls which occur within a /390 application, and on how greatly the /390 application profits from the efficiency of the CISC firmware.
System calls are processed in the server's TPR or SIH processor state (x86-64 mode). The applications run in the TU processor state (/390 mode).
The efficiency of the CISC firmware thus depends on how the JIT buffer is used by the load concerned: on the one hand on the repetition rate of specific /390 code parts (low CPU requirement), and on the other hand on the size of the program parts of a /390 application translated into x86-64 code (higher load on main memory and main memory cache).
CPU performance
The nominal RPF values (see “Standard values for x86 servers”) do not always allow reliable conclusions to be drawn with regard to runtimes, transaction rates or CPU time consumption. Depending on the application, these values can be higher or lower.
The RPF values measured for an x86 server apply for applications with a TU percentage of 40 - 50%. In the case of applications with a TU percentage outside this range, the performance of an application can deviate from the nominal RPF values.
See also the section “Migration from /390 servers to x86 servers”.