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Full Discussion: Byte swap timing
Top Forums Programming Byte swap timing Post 302975261 by migurus on Thursday 9th of June 2016 09:25:45 PM
Old 06-09-2016
Byte swap timing
I have noticed the difference in byte swap timing between two Ubuntu systems. The bswap_32 used to work just fine on the old system, but on the new one it lags behind home-grown swap.

My code:
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <byteswap.h>
#include <sys/time.h>
 union   _U {
          unsigned int          in;
          struct _CH {
                  unsigned char c[4];
          } CH;
};
 int main(int argc, char *argv[])
{
  struct timeval t1, t2;
   union _U u;
  int   n;
  unsigned char tmp;
  
         u.in = (argc == 2) ? atoi(argv[1]) : 0xff;
         gettimeofday(&t1, NULL);
        for(n = 0; n < 100 * 1000000; n++)
        {
                u.in = n;

                 // Version 1
                u.in = bswap_32(u.in);

                 // Version 2 (homegrown)
                //tmp = u.CH.c[0];
                //u.CH.c[0] = u.CH.c[3];
                //u.CH.c[1] = u.CH.c[2];
                //u.CH.c[2] = u.CH.c[1];
                //u.CH.c[3] = tmp;
        }
        gettimeofday(&t2, NULL);
        printf("%lu mls\n", 
                (t2.tv_sec - t1.tv_sec) * 1000000 + (t2.tv_usec - t1.tv_usec));
        return(0);
}

I compile it on both systems simply as
Code:
cc -g

+++

Old system : ProLiant ML350 G4 / Intel(R) Xeon(TM) CPU 3.20GHz (about 8 years old)
Linux OLD 2.6.24-32-server #1 SMP Thu Jul 12 15:21:48 UTC 2012 i686 GNU/Linux
gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu4)

Version 1 avg 500,000 mls, version 2 avg 1,250,000 mls

+++

New system : ProLiant DL360p Gen8 / Intel(R) Xeon(R) CPU E5-2609 0 @ 2.40GHz 4 cores (about 3 years old)
Linux NEW 3.13.0-55-generic #94-Ubuntu SMP Thu Jun 18 00:27:10 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
gcc version 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2)

Version 1 avg 480,000 mls, version 2 avg 390,000 mls

+++

Why on the new system a standard bswap_32 is quite a bit slower than manually shuffling the bytes?

Thanks in advance.
 

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LEARN ABOUT HPUX

allocate_fs_swapmap

allocate_fs_swapmap(5)						File Formats Manual					    allocate_fs_swapmap(5)

NAME

       allocate_fs_swapmap - determines when swapmap structures are allocated for filesystem swap

VALUES

   Failsafe
   Default
   Allowed values
       or

DESCRIPTION

       The tunable is used to determine whether to allocate all needed filesystem swap structures during the initialization of the filesystem swap
       device or wait until they are needed.  Pre-allocation of all filesystem swap structures upon initialization saves time during  later  usage
       of the filesystem swap, and prevents later allocation failure due to physical memory contention.

       Dynamic allocation decreases the memory footprint of the filesystem swap system.

   Who is Expected to Change This Tunable?
       Anyone.

   Restrictions on Changing
       Changes to this tunable take effect at the next reboot.

   When Should the Value of This Tunable Be Raised?
       Systems under heavy memory load may not be able to acquire enough memory to hold the swap mapping structures for an addition of file system
       swap.  These calls to add swap would fail with the error though presumably they would be retried.  Frequent failure  on	file  system  swap
       addition with would merit enabling this tunable.

   What Are the Side Effects of Raising the Value?
       All  the  swap  map structures for each file system swap device would be preallocated during swap initialization at kernel boot, increasing
       the memory footprint of the kernel.  The amount of increase depends on the number and size of file system swap devices.

   When Should the Value of This Tunable Be Lowered?
       Systems with limited memory available to the kernel that also have limited use of file system swap devices should disable this  tunable	to
       decrease kernel memory usage.

   What Are the Side Effects of Lowering the Value?
       File system swap additions may fail with when memory contention is high.

   What Other Tunable Values Should Be Changed at the Same Time?
       None.

WARNINGS

       All  HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases of
       HP-UX.

       Installation of optional kernel software, from HP or other vendors, may cause changes to tunable  parameter  values.   After  installation,
       some  tunable  parameters may no longer be at the default or recommended values.  For information about the effects of installation on tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

							     Tunable Kernel Parameters					    allocate_fs_swapmap(5)
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