Rick Troth wrote:
But if the distributors are only shipping 64-bit kernels, then '-m31'
still does not completely answer the question about 31-bit performance.
The rest of Linux will be running 64-bit, skewing the results.
-- R;
Not necessarily. Granted, the process will be running in z/Architecture
mode, no matter what, but the instructions used will be 31 bit only and
executing in a 31 bit addressing mode. It all depends on the cost of
executing 31 bit instructions in z/Architecture (aka s390x) vs executing
31 bit instructions in ESA/390 mode (aka s390).
In 31 bit addressing mode and using 31 bit (or 32 bit) instructions
behave (almost) exactly the same whether you are running in
z/Architecture or ESA/390 mode of operations. The timing MIGHT be
different depending on the hardware implementation - but you probably
WILL see a difference between that and running a process in 64 bit
addressing mode that is executing 64 bit only instructions.
For example, the instruction to store multiple 32 bit registers (STM)
will behave the same whether you are running in z/Arch or ESA/390 mode
(it will take the registers and store the 32 least significant bits of
those registers (that's 1/2 of it in 64 bit and the whole thing in 32
bit) and store them in consecutive 32 bit locations) - so for that
example, you could see a difference between -m64 and -m31 (even if your
kernel has switched to z/Architecture at IPL).
Also, it's obvious the kernel itself may use some 64 bit instructions
(and thus, on the premise that 64 bit has a different cost than 31 bit)
it will probably impact your user/sys ratio. But for mainly problem
state (aka userland) stuff, you will probably see a difference between
-m64 and -m31 (although it may only affect pointer/long movements)
Again, a program making heavy use of 'long long' will probably suffer a
performance impact while running in 31 bit mode.
Finally, since hardware implementations are already geared towards 64
bit - I'm not even sure the performance penalty is *that* severe.
However, this is a highly speculative statement !
I will concede however that measuring the overall impact might be
another story.
--Ivan
(PS : On a side note.. this is slightly different than running 31 bit
CMS on a 64 bit CP since CP switches the architecture of the dispatched
context of a virtual machine thanks to SIE).
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