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Re: [PATCH] Count number of logical processors sharing L2 cache
- From: Carlos O'Donell <carlos at redhat dot com>
- To: Paul Eggert <eggert at cs dot ucla dot edu>, "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Florian Weimer <fweimer at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 31 May 2016 11:55:12 -0400
- Subject: Re: [PATCH] Count number of logical processors sharing L2 cache
- Authentication-results: sourceware.org; auth=none
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On 05/31/2016 11:44 AM, Paul Eggert wrote:
> On 05/31/2016 08:26 AM, Carlos O'Donell wrote:
>> Most reasonable administrators would use CAT to limit one
>> particular workload and leave the CAT setting until the workload
>> changed.
>
> This sounds like a reasonable assumption. That being said, on fancier
> processors (Intel Xeon E5 v4), the CAT COS tables can change at
> run-time. Plus, even on less-fancy processors I would not be
> surprised if some kernels blindly overwrite a CPU's COSid on context
> switch, so that the last logical thread to switch into a CPU socket
> determines the CAT COS tables for all physical threads currently on
> that socket. Either way, it would be wise to document the assumption,
> as it reminds me of other assumptions (e.g., "the time zone database
> does not change during execution of the program") that formerly were
> quite reasonable but now show signs of strain.
Agreed. I also believe that "the nameservers do not change during
the execution of the program" is a reasonable thing to want (I'm old),
but I have seen a number of cogent arguments against this that I'm
slowly starting to change my mind.
--
Cheers,
Carlos.