From: Ferruh Yigit <ferruh.yigit@intel.com>
To: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>,
thomas@monjalon.net, jerin.jacob@caviumnetworks.com,
techboard@dpdk.org
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH 1/2] eal: add macro to mark variable mostly read only
Date: Wed, 18 Apr 2018 19:03:06 +0100 [thread overview]
Message-ID: <291a43da-6c2d-f65b-374d-206a0f674db6@intel.com> (raw)
In-Reply-To: <20180418175505.GA17954@ltp-pvn>
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
>
>>
>> [1]
>> If this is special section, can you please point counter part in the kernel?
>
> The kernel has something similar[1] but they have a custom linker script to
> arrange symbols.
>
> [1] https://github.com/torvalds/linux/blob/a27fc14219f2e3c4a46ba9177b04d9b52c875532/arch/x86/include/asm/cache.h#L11
> kernel commit id 54cb27a71f51d304342c79e62fd7667f2171062b
>
>>
>>
>>> +
>>> /*********** Macros for pointer arithmetic ********/
>>>
>>> /**
>>> --
>>> 2.17.0
>>>
>>
next prev parent reply other threads:[~2018-04-18 18:03 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 [this message]
2018-04-19 9:20 ` Pavan Nikhilesh
2018-04-19 12:09 ` Bruce Richardson
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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