From: Tejun Heo <tj@kernel.org>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: "Björn Töpel" <bjorn.topel@gmail.com>, bpf <bpf@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"Christoph Lameter" <cl@linux.com>,
"Dennis Zhou" <dennis@kernel.org>
Subject: Re: Percpu variables, benchmarking, and performance weirdness
Date: Fri, 20 Dec 2019 07:12:39 -0800 [thread overview]
Message-ID: <20191220151239.GE2914998@devbig004.ftw2.facebook.com> (raw)
In-Reply-To: <20191220103420.6f9304ab@carbon>
On Fri, Dec 20, 2019 at 10:34:20AM +0100, Jesper Dangaard Brouer wrote:
> > So, my question to the uarch/percpu folks out there: Why are percpu
> > accesses (%gs segment register) more expensive than regular global
> > variables in this scenario.
>
> I'm also VERY interested in knowing the answer to above question!?
> (Adding LKML to reach more people)
No idea. One difference is that percpu accesses are through vmap area
which is mapped using 4k pages while global variable would be accessed
through the fault linear mapping. Maybe you're getting hit by tlb
pressure?
Thanks.
--
tejun
next prev parent reply other threads:[~2019-12-20 15:12 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAJ+HfNgNAzvdBw7gBJTCDQsne-HnWm90H50zNvXBSp4izbwFTA@mail.gmail.com>
2019-12-20 9:34 ` Percpu variables, benchmarking, and performance weirdness Jesper Dangaard Brouer
2019-12-20 15:12 ` Tejun Heo [this message]
2019-12-20 15:36 ` Christopher Lameter
2019-12-20 17:10 ` Dennis Zhou
2019-12-20 16:22 ` Eric Dumazet
2019-12-20 16:34 ` Tejun Heo
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