Mailinglist Archive: opensuse-factory (355 mails)

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Re: [opensuse-factory] compiler options and boinc
On 2020-03-08 18:24, Jan Engelhardt wrote:
On Saturday 2020-03-07 23:16, Frans de Boer wrote:

I noticed that when I compile boinc, it is always around 4x slower on the CPU
benchmarks then the version distributed by Tumbleweed. I do use '-O3
-funroll-loops -ffast-math' as specified on the boinc website.
Did somebody blindly grab something from the manpage without
understanding - and conveying to subsequent readers - the implications?

-funroll-loops:
"This option makes code larger, and may or may not make it run
faster."

(It does not say it explicitly, but "larger" leads to "slower" at a certain
point.)

Yes, after a certain point the benefit of using sequential code can be negative if - for instance - the code runs out the cache, requiring additional memory fetches. And no, it was a man page, but a page on the boinc site itself as well as discussions on various fora.

Note that -ffast-math is even inserted automatically by the build system of boinc itself. Apparently, they do not care so much about precision. Then again, it is only boinc itself and does not affect the external tasks.

But to be sure, I tested various configurations and found that both options do nothing to the issue. In fact, after experimenting with various other benchmark tools, I see a difference of aprox. 7% between two systems. Using the CPU benchmark from the TW boinc distribution, the difference on one machine is 3,8x (AMD) and the other 6,3x (Intel) as compared with the self compiled programs. So, what did the openSUSE team did to get these benchmark results which do not reflect the real world results?

--- Frans.

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