Re: [Lse-tech] Re: (RFC): SKB Initialization

From: Dave Hansen (
Date: Fri Aug 23 2002 - 15:30:07 EST

Bill Hartner wrote:
> Dave Hansen wrote:
>>Mala Anand wrote:
>>>Readprofile ticks are not as accurate as the cycles I measured.
>>>Moreover readprofile can give misleading information as it profiles
>>>on timer interrupts. The alloc_skb and __kfree_skb call memory
>>>management routines and interrupts are disabled in many parts of that code.
>>>So I don't trust the readprofile data.
>>I don't believe your results to be accurate. They may be _precise_
>>for a small case, but you couldn't have been measuring them for very
>>long. A claim of accuracy requires a large number of samples, which
>>you apparently did not do.
> What is your definition of a "very long time" ?
> Read the 1st email. There were 2.4 million samples.
> How many do you think is sufficient ?

I must have misunderstood the data from the first email. I was under
the impression that it was much smaller than that number.

>>I can't use oprofile or other NMI-based profilers on my hardware, so
>>we'll just have to guess. Is there any chance that you have access to
>>a large Specweb setup on hardware that is close to mine and can run
> Why do you think oprofile is a better way to measure this ?

Mala's main complaint about readprofile is that it cannot profile
while interrupts are disabled. oprofile's timer interrupts cannot be
disabled, they _always_ occur.

> BTW, Mala works with Troy Wilson who is running SPECweb99 on
> an 8-way system using Apache. Troy has run with Mala's patch
> and that data will be posted.

I look forward to seeing it.

>>Where are interrupts disabled? I just went through a set of kernprof
>>data and traced up the call graph. In the most common __kfree_skb
>>case, I do not believe that it has interupts disabled. I could be
>>wrong, but I didn't see it.
> What is the revelance of the above ?

Mala's main complaint about readprofile is that it cannot profile
while interrupts are disabled. I didn't see the case where it was
being called with interrupts disabled. I was hoping that you could
point it out to me.

Dave Hansen

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