Re: [patch 7/8] Immediate Values - Documentation

Mathieu Desnoyers
2007-09-21T13:31:29Z

* 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