Re: Measured overhead of timer interrupts

Khimenko Victor (khim@sch57.msk.ru)
Fri, 23 Jul 1999 04:58:35 +0400 (MSD)


In <19990723003639.B3385@fred.muc.de> Andi Kleen (ak@muc.de) wrote:
AK> On Thu, Jul 22, 1999 at 09:12:22PM +0200, Khimenko Victor wrote:
>> In <19990722171109.A1783@fred.muc.de> Andi Kleen (ak@muc.de) wrote:
>> > On Thu, Jul 22, 1999 at 05:02:14PM +0200, Ingo Molnar wrote:
>> >>
>> >> On 22 Jul 1999, Andi Kleen wrote:
>> >>
>> >> > It is a kernel bug, HZ is not exported. Actually two, because none of
>> >> > the /proc should have used it as units, but that cannot be fixed anymore.
>> >>
>> >> why cannot it be fixed?
>>
>> > because that would break the interface.
>>
>> /proc interface are in permanent change. We should keep it consistent in one
>> stable kernel series but I see no reason to keep it consistent between major
>> kernel updates: quite a lot of things will be changed anyway.

AK> This is not really true. There are new columns or lines added to /proc
AK> files, but old ones are not changes, and correctly writen /proc parsers
AK> should not break (unfortunately there are lots of incorrectly writen
AK> proc parser around..)

I yet to see "correct parser" with ability to find out file-max, file-nr,
inode-max and inode0-nr moved from /proc/sys/kernel in 2.0 to
/proc/sys/fs in 2.2...

>> >> > It is easy to add a read-only sysctl for HZ though and make netstat use
>> >> > this, but this won't fix the trillions of other /proc parsers.
>> >>
>> >> we do not want to export HZ, why should we? HZ has no meaning to anything
>> >> else than the kernel. If the kernel exports HZ-dependent values into
>> >> /proc, then that has to be fixed. (yes it might be painful in some cases)
>> >> HZ might even go away in future kernels - what if we start using
>> >> nonperiodic timer interrupts?
>>
>> > Actually, POSIX wants it (sysconf(_SC_CLK_TCK)). glibc currently returns
>> > the HZ value it was compiled with, but that is hardly satisfying.
>>
>> This means that GLiBC must be fixed...

AK> First the kernel needs to export the necessary information. glibc is
AK> hardly to blame here.

Of course. It's two step procedure :-) I do not blame GLibC here, I describe
what must be done...

>> > I agree that the time related sysctls/proc files should be fixed, but I'm
>> > afraid it is too late.
>>
>> /proc files are not a problem but sysctls... In which sysctls HZ is used in
>> such way that GLibC recompilation will not help ?

AK> It has nothing to do with glibc. There are documented sysctls that use
AK> HZ as unit, and Linux has long ago left the innocent state where you
AK> can change documented interfaces without thinking and some migration
AK> strategy.

When it "left the innocent state" ??? I always was sure that there are is
well-defined migration strategy: broke things in experimental branch and
make sure all needed programs will be updated...

Concerning HZ: what are those sysctls ? How many programs use them ?

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