How the util of iostat is computed?

Question

iostat -x -d 

can display many i/o statistic info. For util of iostat, the explanation is :

Percentage of CPU time during which I/O requests were issued to the device (band-width utilization for the device). Device saturation occurs when this value is close to 100%

I want to know how the util was computed?

I make an experiment, (see following code), start 40 thread to randomly read 40 files. I suppose the disk util should be very high, but I am wrong, the iostat is as follow, anyone can give why? THX

Device:         rrqm/s   wrqm/s   r/s   w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sdb1              0.01     0.44  0.24  0.57     3.44     8.14    14.34     0.00    2.28   0.66   0.05

Code:

#include <iostream>
#include <fstream>
#include <pthread.h>

using namespace std;

void* work(void* a)
{
    int* id = (int*)a;
    string file = "sys.partition";
    char buf[100];
    sprintf(buf, "%d", *id);
    file.append(string(buf));
    ifstream in(file.c_str());
    in.seekg(0, ios_base::end);
    size_t len = in.tellg();

    cout << "open file : " << file << " , " << len << endl;
    srand(time(NULL));

    while(true)
    {
        size_t pos = rand() % len;
        in.seekg(pos);
        //cout << pos << endl;
        in.read(buf, 10);
        system("sync");
    }
    in.close();
}

int main(int argc, char** argv)
{
    static const int num = 40;
    pthread_t threads[num];
    for (int i = 0; i < num; i++)       {
        pthread_create(&threads[i], NULL, work, &i);
    }
    for (int i = 0; i < num; i++)       {
        pthread_join(threads[i], NULL);
    }
    return 0;
}

Answer 1

%util is named busy in the source code of iostat: https://code.google.com/p/tester-higkoo/source/browse/trunk/Tools/iostat/iostat.c#380

Busy is counted as percent ratio of Ticks to deltams, limited to 100%

busy = 100.0 * blkio.ticks / deltams; /* percentage! */
if (busy > 100.0) busy = 100.0;

DeltaMS is sum of system load for the period of time (user time + system time + idle time + iowait)/ ncpu.

double deltams = 1000.0 *
        ((new_cpu.user + new_cpu.system +
          new_cpu.idle + new_cpu.iowait) -
         (old_cpu.user + old_cpu.system +
          old_cpu.idle + old_cpu.iowait)) / ncpu / HZ;

Ticks - is the Time of requests in queue for the period

blkio.ticks = new_blkio[p].ticks
                - old_blkio[p].ticks;

In more current version of sysstat the code is bit different: http://sources.debian.net/src/sysstat/10.2.0-1/iostat.c#L959

/*       rrq/s wrq/s   r/s   w/s  rsec  wsec  rqsz  qusz await r_await w_await svctm %util */
printf(" %8.2f %8.2f %7.2f %7.2f %8.2f %8.2f %8.2f %8.2f %7.2f %7.2f %7.2f %6.2f %6.2f\n",
...
       /*
        * Again: Ticks in milliseconds.
        * In the case of a device group (option -g), shi->used is the number of
        * devices in the group. Else shi->used equals 1.
        */
       shi->used ? xds.util / 10.0 / (double) shi->used
                 : xds.util / 10.0);    /* shi->used should never be null here */

xds is filled in the compute_ext_disk_stats(&sdc, &sdp, itv, &xds); http://sources.debian.net/src/sysstat/10.2.0-1/common.c?hl=679#L679

/*
 * Macros used to display statistics values.
 *
 * HZ is 1024 on IA64 and % should be normalized to 100.
 */
#define S_VALUE(m,n,p)  (((double) ((n) - (m))) / (p) * HZ)

xds->util  = S_VALUE(sdp->tot_ticks, sdc->tot_ticks, itv);

And there is the filling of tot_ticks from iostat.c

  * @ioi        Current sample statistics.
  * @ioj        Previous sample statistics.
  * @itv        Interval of time.
  ...

sdc.tot_ticks = ioi->tot_ticks;
sdp.tot_ticks = ioj->tot_ticks;

tot_ticks are read from "sysfs stat for current block device or partition" in read_sysfs_file_stat (iostat.c:487), and ioi and ioj are current and previous stat.