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Re: [PATCH] Count number of logical processors sharing L2 cache
- From: Paul Eggert <eggert at cs dot ucla dot edu>
- To: Carlos O'Donell <carlos at redhat dot com>, "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 08:44:41 -0700
- 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 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.