wrong testing result of mbw

[Wei Liu]

The testing result of mbw tool installed by running “apt install mbw” is shown below.

(tf2) bruce@P800002:~/tools/memtest/mbw_original$ mbw -a -n 1 1024

Long uses 8 bytes. Allocating 2*134217728 elements = 2147483648 bytes of memory.

Using 262144 bytes as blocks for memcpy block copy test.

Getting down to business... Doing 1 runs per test.

0 Method: MEMCPY Elapsed: 0.20496 MiB: 1024.00000 Copy: 4996.097 MiB/s

0 Method: DUMB Elapsed: 0.11309 MiB: 1024.00000 Copy: 9055.136 MiB/s

0 Method: MCBLOCK Elapsed: 0.18077 MiB: 1024.00000 Copy: 5664.625 MiB/s

But the testing result from newest version of mbw which is downloaded from github and built by myself is totally different with the former one.

(tf2) bruce@P800002:~/tools/memtest/mbw_original$ ./mbw -a -n 1 1024

Long uses 8 bytes. Allocating 2*134217728 elements = 2147483648 bytes of memory.

Using 262144 bytes as blocks for memcpy block copy test.

Getting down to business... Doing 1 runs per test.

0 Method: MEMCPY Elapsed: 0.11314 MiB: 1024.00000 Copy: 9050.334 MiB/s

0 Method: DUMB Elapsed: 0.20412 MiB: 1024.00000 Copy: 5016.632 MiB/s

0 Method: MCBLOCK Elapsed: 0.22396 MiB: 1024.00000 Copy: 4572.225 MiB/s

From the output of self-built mbw, you can see the speed of MEMCPY is about 9050.334 MiB/s (much higher than the case of DUMB), but it is only 4996.097 MiB/s in the output of apt-installed mbw (much lower than the case of DUMB).

After reading output of “git log -p mbw.c”, I believe apt repository is still using a very old version of mbw, and unfortunately this version involves a seriously issue which was fixed in the following changeset.

In this changeset, the code “if(type==1)” was changed to “if(type==TEST_MEMCPY)” and the micro TEST_MEMCPY is 0. That means when the “help” information(“-t1: dumb (b[i]=a[i] style) test”) shows number 1 case is DUMB, what it really did is MEMCPY.

Looking forward to fix this problem in apt repository by updating to newest mbw in github.

diff --git a/mbw.c b/mbw.c

index 6251ea9..06a9d09 100644

--- a/mbw.c

+++ b/mbw.c

@@ -23,6 +23,11 @@

/* default block size for test 2, in bytes */

#define DEFAULT_BLOCK_SIZE 262144

+/* test types */

+#define TEST_MEMCPY 0

+#define TEST_DUMB 1

+#define TEST_MCBLOCK 2

+

/*

  * MBW memory bandwidth benchmark

  *

@@ -50,9 +55,9 @@ void usage()

     printf("Options:\n");

     printf(" -n: number of runs per test\n");

     printf(" -a: Don't display average\n");

- printf(" -t0: memcpy test\n");

- printf(" -t1: dumb (b[i]=a[i] style) test\n");

- printf(" -t2 : memcpy test with fixed block size\n");

+ printf(" -t%d: memcpy test\n", TEST_MEMCPY);

+ printf(" -t%d: dumb (b[i]=a[i] style) test\n", TEST_DUMB);

+ printf(" -t%d: memcpy test with fixed block size\n", TEST_MCBLOCK);

     printf(" -b <size>: block size in bytes for -t2 (default: %d)\n", DEFAULT_BLOCK_SIZE);

     printf(" -q: quiet (print statistics only)\n");

     printf("(will then use two arrays, watch out for swapping)\n");

@@ -100,13 +105,13 @@ double worker(unsigned long long asize, long *a, long *b, int type, unsigned lon

     /* array size in bytes */

     unsigned long long array_bytes=asize*long_size;

- if(type==1) { /* memcpy test */

+ if(type==TEST_MEMCPY) { /* memcpy test */

         /* timer starts */

         gettimeofday(&starttime, NULL);

         memcpy(b, a, array_bytes);

         /* timer stops */

         gettimeofday(&endtime, NULL);

- } else if(type==2) { /* memcpy block test */

+ } else if(type==TEST_MCBLOCK) { /* memcpy block test */

         char* aa = (char*)a;

         char* bb = (char*)b;

         gettimeofday(&starttime, NULL);

@@ -117,7 +122,7 @@ double worker(unsigned long long asize, long *a, long *b, int type, unsigned lon

             bb=mempcpy(bb, aa, t);

         }

         gettimeofday(&endtime, NULL);

- } else { /* dumb test */

+ } else if(type==TEST_DUMB) { /* dumb test */

         gettimeofday(&starttime, NULL);

         for(t=0; t<asize; t++) {

             b[t]=a[t];

@@ -142,13 +147,13 @@ double worker(unsigned long long asize, long *a, long *b, int type, unsigned lon

void printout(double te, double mt, int type)

{

     switch(type) {

- case 0:

+ case TEST_MEMCPY:

             printf("Method: MEMCPY\t");

             break;

- case 1:

+ case TEST_DUMB:

             printf("Method: DUMB\t");

             break;

- case 2:

+ case TEST_MCBLOCK:

             printf("Method: MCBLOCK\t");

             break;

     }

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: mbw 1.2.2-1build1
Uname: Linux 5.8.1inspurp8000 x86_64
ApportVersion: 2.20.9-0ubuntu7.24
Architecture: amd64
Date: Fri Jul 23 14:15:17 2021
Dependencies:
 gcc-8-base 8.4.0-1ubuntu1~18.04
 libc6 2.27-3ubuntu1.4
 libgcc1 1:8.4.0-1ubuntu1~18.04
InstallationDate: Installed on 2021-07-07 (15 days ago)
InstallationMedia: Ubuntu 18.04.5 LTS "Bionic Beaver" - Release amd64 (20200806.1)
SourcePackage: mbw
UpgradeStatus: No upgrade log present (probably fresh install)