Re: Intel P6 vs P7 system call performance

[Pavel Machek]

Hi!

> I've created a modified glibc which uses the syscall code for almost
> everything.  There are a few int $0x80 left here and there but mostly it
> is a centralized change.
> 
> The result: all works as expected.  Nice.
> 
> On my test machine your little test program performs the syscalls on
> roughly twice as fast (HT P4, pretty new).  Your numbers are perhaps for
> the P4 Xeons.  Anyway, when measuring some more involved code (I ran my
> thread benchmark) I got only about 3% performance increase.  It's doing
> a fair amount of system calls.  But again, the good news is your code
> survived even this stress test.
> 
> 
> The problem with the current solution is the instruction set of the x86.
>  In your test code you simply use call 0xfffff000 and it magically work.
>  But this is only the case because your program is linked statically.
> 
> For the libc DSO I had to play some dirty tricks.  The x86 CPU has no
> absolute call.  The variant with an immediate parameter is a relative
> jump.  Only when jumping through a register or memory location is it
> possible to jump to an absolute address.  To be clear, if I have
> 
>     call 0xfffff000
> 
> in a DSO which is loaded at address 0x80000000 the jumps ends at
> 0x7fffffff.  The problem is that the static linker doesn't know the load
> address.  We could of course have the dynamic linker fix up the
> addresses but this is plain stupid.  It would mean fixing up a lot of
> places and making of those pages covered non-sharable.

Can't you do call far __SOME_CS, 0xfffff000?

								Pavel
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