From: Bruce Richardson <bruce.richardson@intel.com>
To: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
Cc: Ferruh Yigit <ferruh.yigit@intel.com>,
thomas@monjalon.net, jerin.jacob@caviumnetworks.com,
techboard@dpdk.org, dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH 1/2] eal: add macro to mark variable mostly read only
Date: Thu, 19 Apr 2018 13:09:58 +0100 [thread overview]
Message-ID: <20180419120958.GC11352@bricha3-MOBL.ger.corp.intel.com> (raw)
In-Reply-To: <20180419092051.GA8072@ltp-pvn>
On Thu, Apr 19, 2018 at 02:50:52PM +0530, Pavan Nikhilesh wrote:
> On Wed, Apr 18, 2018 at 07:03:06PM +0100, Ferruh Yigit wrote:
> > On 4/18/2018 6:55 PM, Pavan Nikhilesh wrote:
> > > On Wed, Apr 18, 2018 at 06:43:11PM +0100, Ferruh Yigit wrote:
> > >> On 4/18/2018 4:30 PM, Pavan Nikhilesh wrote:
> > >>> Add macro to mark a variable to be mostly read only and place it in a
> > >>> separate section.
> > >>>
> > >>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
> > >>> ---
> > >>>
> > >>> Group together mostly read only data to avoid cacheline bouncing, also
> > >>> useful for auditing purposes.
> > >>>
> > >>> lib/librte_eal/common/include/rte_common.h | 5 +++++
> > >>> 1 file changed, 5 insertions(+)
> > >>>
> > >>> diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
> > >>> index 6c5bc5a76..f2ff2e9e6 100644
> > >>> --- a/lib/librte_eal/common/include/rte_common.h
> > >>> +++ b/lib/librte_eal/common/include/rte_common.h
> > >>> @@ -114,6 +114,11 @@ static void __attribute__((constructor(prio), used)) func(void)
> > >>> */
> > >>> #define __rte_noinline __attribute__((noinline))
> > >>>
> > >>> +/**
> > >>> + * Mark a variable to be mostly read only and place it in a separate section.
> > >>> + */
> > >>> +#define __rte_read_mostly __attribute__((__section__(".read_mostly")))
> > >>
> > >
> > > Hi Ferruh,
> > >
> > >> Hi Pavan,
> > >>
> > >> Is the section ".read_mostly" treated specially [1] or is this just for grouping
> > >> symbols together (to reduce cacheline bouncing)?
> > >
> > > The section .read_mostly is not treated specially it's just for grouping
> > > symbols.
> >
> > I have encounter with a blog post claiming this is not working:
> >
> > "
> > The problem with the above approach is that once all the __read_mostly variables
> > are grouped into one section, the remaining "non-read-mostly" variables end-up
> > together too. This increases the chances that two frequently used elements (in
> > the "non-read-mostly" region) will end-up competing for the same position (or
> > cache-line, the basic fixed-sized block for memory<-->cache transfers) in the
> > cache. Thus frequent accesses will cause excessive cache thrashing on that
> > particular cache-line thereby degrading the overall system performance.
> > "
> >
> > https://thecodeartist.blogspot.com/2011/12/why-readmostly-does-not-work-as-it.html
> >
>
> The author is concerned about processors with less cache set-associativity,
> almost all modern processors have >= 16 way set associativity. And the above
> issue can happen even now when two frequently written global variables are
> placed next to each other.
>
> Currently, we don't have much control over how the global variables are
> arranged and a single addition/deletion to the global variables causes change
> in alignment and in some cases minor performance regression.
> Tagging them as __read_mostly we can easily identify the alignment changes
> across builds by comparing map files global variable section.
>
> I have verified the patch-set on arm64 (16-way set-associative) and didn't
> notice any performance regression.
> Did you have a chance to verify if there is any performance regression?
>
Is there a performance improvement? It's seems a relatively strange change
to me, so I'd like to know that it really improves performance in test
cases.
/Bruce
next prev parent reply other threads:[~2018-04-19 12:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-18 15:30 Pavan Nikhilesh
2018-04-18 15:30 ` [dpdk-dev] [PATCH 2/2] drivers: mark logtype variables as read mostly Pavan Nikhilesh
2018-04-18 17:43 ` [dpdk-dev] [PATCH 1/2] eal: add macro to mark variable mostly read only Ferruh Yigit
2018-04-18 17:55 ` Pavan Nikhilesh
2018-04-18 18:03 ` Ferruh Yigit
2018-04-19 9:20 ` Pavan Nikhilesh
2018-04-19 12:09 ` Bruce Richardson [this message]
2018-04-19 15:18 ` Pavan Nikhilesh
2018-04-19 15:37 ` Bruce Richardson
2018-04-19 15:55 ` Pavan Nikhilesh
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