linux-kernel - Re: [PATCH 06/14] Pramfs: Include files

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Date:	Mon, 22 Jun 2009 23:41:55 +0200
From:	Jörn Engel <joern@...fs.org>
To:	Chris Simmonds <chris@...t.co.uk>
Cc:	Arnd Bergmann <arnd@...db.de>, Marco <marco.stornelli@...il.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/14] Pramfs: Include files

On Mon, 22 June 2009 20:31:10 +0100, Chris Simmonds wrote:
> 
> I disagree: that adds an unnecessary overhead for those architectures 
> where the cpu byte order does not match the data structure ordering. I 
> think the data structures should be native endian and when mkpramfs is 
> written it can take a flag (e.g. -r) in the same way mkcramfs does.

Just to quantify this point, I've written a small crap program:
#include <stdio.h>
#include <stdint.h>
#include <byteswap.h>
#include <sys/time.h>

long long delta(struct timeval *t1, struct timeval *t2)
{
	long long delta;

	delta  = 1000000ull * t2->tv_sec + t2->tv_usec;
	delta -= 1000000ull * t1->tv_sec + t1->tv_usec;
	return delta;
}

#define LOOPS 100000000
int main(void)
{
	long native = 0;
	uint32_t narrow = 0;
	uint64_t wide = 0, native_wide = 0;
	struct timeval t1, t2, t3, t4, t5;
	int i;

	gettimeofday(&t1, NULL);
	for (i = 0; i < LOOPS; i++)
		native++;
	gettimeofday(&t2, NULL);
	for (i = 0; i < LOOPS; i++)
		narrow = bswap_32(bswap_64(narrow) + 1);
	gettimeofday(&t3, NULL);
	for (i = 0; i < LOOPS; i++)
		native_wide++;
	gettimeofday(&t4, NULL);
	for (i = 0; i < LOOPS; i++)
		wide = bswap_64(bswap_64(wide) + 1);
	gettimeofday(&t5, NULL);
	printf("long:  %9lld us\n", delta(&t1, &t2));
	printf("we32:  %9lld us\n", delta(&t2, &t3));
	printf("u64:   %9lld us\n", delta(&t3, &t4));
	printf("we64:  %9lld us\n", delta(&t4, &t5));
	printf("loops: %9d\n", LOOPS);
	return 0;
}

Four loops doing the same increment with different data types: long,
u64, we32 (wrong-endian) and we64.  Compile with _no_ optimizations.

Results on my i386 notebook:
long:     453953 us
we32:     880273 us
u64:      504214 us
we64:    2259953 us
loops: 100000000

Or thereabouts, not completely stable.  Increasing the data width is 10%
slower, 32bit endianness conversions is 2x slower, 64bit conversion is
5x slower.

However, even the we64 loop still munches through 353MB/s (100M
conversions in 2.2s, 8bytes per converion.  Double the number if you
count both conversion to/from wrong endianness).  Elsewhere in this
thread someone claimed the filesystem peaks out at 13MB/s.  One might
further note that only filesystem metadata has to go through endianness
conversion, so on this particular machine it is completely lost in the
noise.

Feel free to run the program on any machine you care about.  If you get
numbers to back up your position, I'm willing to be convinced.  Until
then, I consider the alleged overhead of endianness conversion a prime
example of premature optimization.

Jörn

-- 
Joern's library part 7:
http://www.usenix.org/publications/library/proceedings/neworl/full_papers/mckusick.a
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