linux-kernel - Re: [PATCH 0/9] x86: Concurrent TLB flushes and other improvements

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Message-ID: <fcfeac95-b38c-5ec6-4fd9-9d7931d5ae2e@intel.com>
Date:   Tue, 25 Jun 2019 15:02:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 0/9] x86: Concurrent TLB flushes and other improvements

On 6/12/19 11:48 PM, Nadav Amit wrote:
> Running sysbench on dax w/emulated-pmem, write-cache disabled, and
> various mitigations (PTI, Spectre, MDS) disabled on Haswell:
> 
>  sysbench fileio --file-total-size=3G --file-test-mode=rndwr \
>   --file-io-mode=mmap --threads=4 --file-fsync-mode=fdatasync run
> 
> 			events (avg/stddev)
> 			-------------------
>   5.2-rc3:		1247669.0000/16075.39
>   +patchset:		1290607.0000/13617.56 (+3.4%)

Why did you decide on disabling the side-channel mitigations?  While
they make things slower, they're also going to be with us for a while,
so they really are part of real-world testing IMNHO.  I'd be curious
whether this set has more or less of an advantage when all the
mitigations are on.

Also, why only 4 threads?  Does this set help most when using a moderate
number of threads since the local and remote cost are (relatively) close
vs. a large system where doing lots of remote flushes is *way* more
time-consuming than a local flush?