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From: "Ryan S. Arnold" <ryan.arnold@gmail.com>
To: "Ondřej Bílka" <neleai@seznam.cz>
Cc: Siddhesh Poyarekar <siddhesh@redhat.com>,
	"Carlos O'Donell" <carlos@redhat.com>,
		Will Newton <will.newton@linaro.org>,
		"libc-ports@sourceware.org" <libc-ports@sourceware.org>,
	Patch Tracking <patches@linaro.org>
Subject: Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.
Date: Wed, 04 Sep 2013 17:37:00 -0000	[thread overview]
Message-ID: <CAAKybw8L6A7RpMzbp3WheVciMwMTWko3uWgxV_9KPYtEJZ=WHQ@mail.gmail.com> (raw)
In-Reply-To: <20130904110333.GA6216@domone.kolej.mff.cuni.cz>

On Wed, Sep 4, 2013 at 6:03 AM, Ondřej Bílka <neleai@seznam.cz> wrote:
> On Wed, Sep 04, 2013 at 01:00:09PM +0530, Siddhesh Poyarekar wrote:
>> 2. Scale with size
> Not very important for several reasons. One is that big sizes are cold
> (just look in oprofile output that loops are less frequent than header.)
>
> Second reason is that if we look at caller large sizes are unlikely
> bottleneck.

From my experience, extremely large data sizes are not very common.
Optimizing for those gets diminishing returns.  I believe that at very
large sizes the pressure is all on the hardware anyway.  Prefetching
large amounts of data in a loop takes a fixed amount of time and given
a large enough amount of data, the overhead introduced by most other
factors is negligible.

>> 4. Measure the effect of dcache pressure on function performance
>> 5. Measure effect of icache pressure on function performance.
>>
> Here you really need to base weigths on function usage patterns.
> A bigger code size is acceptable for functions that are called more
> often. You need to see distribution of how are calls clustered to get
> full picture. A strcmp is least sensitive to icache concerns, as when it
> is called its mostly 100 times over in tight loop so size is not big issue.
> If same number of call is uniformnly spread through program we need
> stricter criteria.

Icache pressure is probably one of the more difficult things to
measure with a benchmark.  I suppose it'd be easier with a pipeline
analyzer.

Can you explain how usage pattern analysis might reveal icache pressure?

I'm not sure how useful 'usage pattern' are when considering dcache
pressure.  On Power we have data-cache prefetch instructions and since
we know that dcache pressure is a reality, we will prefetch if our
data sizes are large enough to out-weigh the overhead of prefetching,
e.g., when the data size exceeds the cacheline size.

Ryan

  parent reply	other threads:[~2013-09-04 17:37 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-08-12  7:55 Will Newton
2013-08-27  7:46 ` Will Newton
2013-08-30 17:14   ` Carlos O'Donell
2013-08-30 18:48     ` Will Newton
2013-08-30 19:26       ` Carlos O'Donell
2013-09-02 14:18         ` Will Newton
2013-09-03 16:14           ` Carlos O'Donell
     [not found]         ` <CANu=DmhA9QvSe6RS72Db2P=yyjC72fsE8d4QZKHEcNiwqxNMvw@mail.gmail.com>
2013-09-02 14:18           ` benchmark improvements (Was: Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.) Siddhesh Poyarekar
2013-09-03 13:46             ` Will Newton
2013-09-03 17:48               ` Ondřej Bílka
2013-09-02 19:57           ` [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance Ondřej Bílka
2013-09-03 16:18           ` Carlos O'Donell
2013-09-03 17:37             ` Ondřej Bílka
2013-09-03 17:52               ` Carlos O'Donell
2013-09-03 18:57                 ` Ondřej Bílka
2013-09-03 19:15                   ` Carlos O'Donell
2013-09-04  7:27                     ` Siddhesh Poyarekar
2013-09-04 11:03                       ` Ondřej Bílka
2013-09-04 11:43                         ` Siddhesh Poyarekar
2013-09-04 17:37                         ` Ryan S. Arnold [this message]
2013-09-05  8:04                           ` Ondřej Bílka
2013-09-04 15:30                       ` Carlos O'Donell
2013-09-04 17:35                       ` Ryan S. Arnold
2013-09-05 11:07                         ` Ondřej Bílka
2013-09-05 11:54                         ` Joseph S. Myers
2013-09-03 19:34               ` Ryan S. Arnold
2013-09-07 11:55                 ` Ondřej Bílka
2013-09-03 19:31             ` Ryan S. Arnold
2013-09-03 19:54               ` Carlos O'Donell
2013-09-03 20:56                 ` Ryan S. Arnold
2013-09-03 23:29                   ` Ondřej Bílka
2013-09-03 23:31                   ` Carlos O'Donell
2013-09-03 22:27               ` Ondřej Bílka
2013-08-29 23:58 ` Joseph S. Myers
2013-08-30 14:56   ` Will Newton
2013-08-30 15:18     ` Joseph S. Myers
2013-08-30 18:46       ` Will Newton

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