From: Eric Dumazet <eric.dumazet@gmail.com>
To: Tejun Heo <tj@kernel.org>, 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 08:22:02 -0800 [thread overview]
Message-ID: <a66e79b1-41a8-08f6-8dc2-37ce7a5fff53@gmail.com> (raw)
In-Reply-To: <20191220151239.GE2914998@devbig004.ftw2.facebook.com>
On 12/20/19 7:12 AM, Tejun Heo wrote:
> 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?
I definitely seen expensive per-cpu updates in the stack.
(SNMP counters, or per-cpu stats for packets/bytes counters)
It might be nice to have an option to use 2M pages.
(I recall sending some patches in the past about using high-order pages for vmalloc,
but this went nowhere)
next prev parent reply other threads:[~2019-12-20 16:22 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 ` Jesper Dangaard Brouer
2019-12-20 15:12 ` Tejun Heo
2019-12-20 15:36 ` Christopher Lameter
2019-12-20 17:10 ` Dennis Zhou
2019-12-20 16:22 ` Eric Dumazet [this message]
2019-12-20 16:34 ` Tejun Heo
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