This is the mail archive of the
mailing list for the glibc project.
Re: [PATCH] Count number of logical processors sharing L2 cache
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: "Carlos O'Donell" <carlos at redhat dot com>
- Cc: Florian Weimer <fweimer at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 31 May 2016 07:57:02 -0700
- Subject: Re: [PATCH] Count number of logical processors sharing L2 cache
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOoy2YaQTdyqZpQ3=ytDc5dywNshzHAN2ymN60=L5KwbiA at mail dot gmail dot com> <CAMe9rOoq8MNkX0GvoePQ-C51mfUr2ikrRJgqCZE0CoGoJEmOOw at mail dot gmail dot com> <d4cf36ee-f402-41fe-5108-e072b47f2399 at redhat dot com> <CAMe9rOpUuYboLH9WgyHH4HiBaSBXJ+uB=MPUft2S26N+wYJ9-A at mail dot gmail dot com> <76801b5c-7770-23a9-9b7c-4e44722247e1 at redhat dot com> <CAMe9rOqAuxZZ=gpd1zXvbRrsqjhT8G6C9WBbpwaqa65s=-ZTnQ at mail dot gmail dot com> <57449424 dot 1000009 at redhat dot com> <CAMe9rOpzKgS_uNz1ZFg-sUxCbuY4tFOy19eLjyFaO7UbNYUr1g at mail dot gmail dot com> <574D994F dot 5050207 at redhat dot com>
On Tue, May 31, 2016 at 7:01 AM, Carlos O'Donell <email@example.com> wrote:
> On 05/24/2016 05:35 PM, H.J. Lu wrote:
>> CAT dedicates part of L3 cache to certain processor or process so
>> that L3 cache is always available to them. Glibc tries not to take all
>> L3 cache in memcpy/memset so thar L3 cache is available for other
>> operations within the same process as well as to other processor/process.
>> CAT and glibc work at different angels. There is no direct conflict between
>> CAT and glibc. At the moment, I am not sure if CAT-aware glibc will
>> improve performance.
> What do you mean by "no direct conflict?"
> If glibc tuns its own algorithms to use 1/4 of L3, but CAT has only
> allocated 1/5 of L3 to that process, then glibc's algoirthms, whose
> intent was to use a small amount of L3 are now using *more* L3 than
> the entire process has and that could impact performance?
Cache sizes are only used for instructions selections in
string/memory functions, which don't use cache directly.
Glibc has NO control whatsoever on how much cache
string/memory functions use.
> Did I miss something?
> There could be opportunities for incorrect tunings if glibc is not
Since CAT can change at any time during the life of a process,
checking CAT inside glibc isn't very practical.