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Message-ID: <558C5342.9020702@suse.cz>
Date: Thu, 25 Jun 2015 21:15:14 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>,
linux-mm <linux-mm@...ck.org>, Borislav Petkov <bp@...en8.de>,
Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Minchan Kim <minchan@...nel.org>,
Rik van Riel <riel@...hat.com>,
"Kirill A. Shutemov" <kirill@...temov.name>,
H Peter Anvin <hpa@...or.com>,
Andi Kleen <andi@...stfloor.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Dave Hansen <dave.hansen@...el.com>,
Mel Gorman <mgorman@...e.de>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] TLB flush multiple pages per IPI v5
On 25.6.2015 20:36, Linus Torvalds wrote:
>
> On Jun 25, 2015 04:48, "Ingo Molnar" <mingo@...nel.org
> <mailto:mingo@...nel.org>> wrote:
>>
>> - 1x, 2x, 3x, 4x means up to 4 adjacent 4K vmalloc()-ed pages are accessed, the
>> first byte in each
>
> So that test is a bit unfair. From previous timing of Intel TLB fills, I can
> tell you that Intel is particularly good at doing adjacent entries.
>
> That's independent of the fact that page tables have very good locality (if they
> are the radix tree type - the hashed page tables that ppc uses are shit). So
> when filling adjacent entries, you take the cache misses for the page tables
> only once, but even aside from that, Intel send to do particularly well at the
> "next page" TLB fill case
AFAIK that's because they also cache partial translations, so if the first 3
levels are the same (as they mostly are for the "next page" scenario) it will
only have to look at the last level of pages tables. AMD does that too.
> Now, I think that's a reasonably common case, and I'm not saying that it's
> unfair to compare for that reason, but it does highlight the good case for TLB
> walking.
>
> So I would suggest you highlight the bad case too: use invlpg to invalidate
> *one* TLB entry, and then walk four non-adjacent entries. And compare *that* to
> the full TLB flush.
>
> Now, I happen to still believe in the full flush, but let's not pick benchmarks
> that might not show the advantages of the finer granularity.
>
> Linus
>
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