Messages in this thread | | | Date | Fri, 21 Sep 2007 09:31:03 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 7/8] Immediate Values - Documentation |
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* Denys Vlasenko (vda.linux@googlemail.com) wrote: > On Monday 27 August 2007 16:59, Mathieu Desnoyers wrote: > > +We can therefore affirm that adding 2 markers to getppid, on a system with high > > +memory pressure, would have a performance hit of at least 6.0% on the system > > +call time, all within the uncertainty limits of these tests. The same applies to > > +other kernel code paths. The smaller those code paths are, the highest the > > +impact ratio will be. > > Immediates make code bigger, right?
Nope.
Example:
char x;
void testb(void) { if (x > 5) testa(); }
Would turn into: 56: b0 00 mov $0x0,%al 58: 3c 05 cmp $0x5,%al 5a: 7e 05 jle 61 <testb+0x11>
(6 bytes)
Rather than:
56: 80 3d 00 00 00 00 05 cmpb $0x5,0x0 5d: 7e 05 jle 64 <testb+0x14>
(9 bytes)
So actually, immediate values well used make the code smaller. By the way, I recommend using the smallest immediate values required, which will often be a single byte.
> What will happen on a system with high *icache* pressure?
It *helps* :) And by the way, icache on recent x86 and x86_64 is a trace cache, so I don't see your point anyway.
> There a lot of inline happy and/or C++ folks out there > in the userland, they routinely have programs in tens of megabytes range. > > getppid is one of the lightest syscalls out there. > What kind of speedup do you see on a real-world test > (two processes exchaging data through pipes, for example)? >
With the size of the caches we currently have, that kind of workload will not show any measurable difference: the signal/noise ratio is way to small to detect that kind of performance difference under such workload. Try it if you want.
The real-world speedup I am interested into is to have almost -zero- tracer impact, which imples being undetectable even in the smallest and shortest functions. I guess nobody is interested in adding a measurable performance hit to kmalloc fast path, right ?
> > +Therefore, not only is it interesting to use the immediate values to dynamically > > +activate dormant code such as the markers, but I think it should also be > > +considered as a replacement for many of the "read mostly" static variables. > > What effect that will have on "size vmlinux" on AMD64?
Without considering kernel/immediate.o, it will make the code smaller and add 3*8bytes=24bytes of data in the __immediate section per immediate value reference (data only used for updates).
Mathieu
> -- > vda
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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