linux-kernel - Re: [v3 0/9] parallelized "struct page" zeroing

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Date:   Thu, 11 May 2017 16:47:05 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net
Subject: Re: [v3 0/9] parallelized "struct page" zeroing

>>
>> Have you measured that? I do not think it would be super hard to
>> measure. I would be quite surprised if this added much if anything at
>> all as the whole struct page should be in the cache line already. We do
>> set reference count and other struct members. Almost nobody should be
>> looking at our page at this time and stealing the cache line. On the
>> other hand a large memcpy will basically wipe everything away from the
>> cpu cache. Or am I missing something?
>>

Here is data for single thread (deferred struct page init is disabled):

Intel CPU E7-8895 v3 @ 2.60GHz  1T memory
-----------------------------------------
time to memset "struct pages in memblock: 11.28s
time to init "struct pag"es:               4.90s

Moving memset into __init_single_page()
time to init and memset "struct page"es:   8.39s

SPARC M6 @ 3600 MHz  1T memory
-----------------------------------------
time to memset "struct pages in memblock:  1.60s
time to init "struct pag"es:               3.37s

Moving memset into __init_single_page()
time to init and memset "struct page"es:  12.99s


So, moving memset() into __init_single_page() benefits Intel. I am 
actually surprised why memset() is so slow on intel when it is called 
from memblock. But, hurts SPARC, I guess these membars at the end of 
memset() kills the performance.

Also, when looking at these values, remeber that Intel has twice as many 
"struct page" for the same amount of memory.

Pasha