Normalize DiskIOMeter bar by number of disks

[Explorer09]

The maximum utilization should be "100% * number of disks". Set the bar and graph of the meter to draw with that maximum. Also remove the 100% cap on the utilization percentage display.

Resolves #1374.

[natoscott]

Something like this patch will provide the PCP value you seek. You might want to consider a similar change for the NetworkIOMeter, in which case there is a "hinv.ninterface" metric that can be used similarly.

pcp/Metric.h
--- /tmp/git-blob-5igpu5/Metric.h	2024-09-06 15:32:13.776523149 +1000
+++ pcp/Metric.h	2024-09-06 15:26:46.052772959 +1000
@@ -26,6 +26,7 @@ typedef enum Metric_ {
    PCP_CONTROL_THREADS,         /* proc.control.perclient.threads */
 
    PCP_HINV_NCPU,               /* hinv.ncpu */
+   PCP_HINV_NDISK,              /* hinv.ndisk */
    PCP_HINV_CPUCLOCK,           /* hinv.cpu.clock */
    PCP_UNAME_SYSNAME,           /* kernel.uname.sysname */
    PCP_UNAME_RELEASE,           /* kernel.uname.release */
pcp/Platform.c
--- /tmp/git-blob-JCaR3p/Platform.c	2024-09-06 15:32:13.956523525 +1000
+++ pcp/Platform.c	2024-09-06 15:30:56.940361942 +1000
@@ -127,6 +127,7 @@ static const char* Platform_metricNames[
    [PCP_CONTROL_THREADS] = "proc.control.perclient.threads",
 
    [PCP_HINV_NCPU] = "hinv.ncpu",
+   [PCP_HINV_NDISK] = "hinv.ndisk",
    [PCP_HINV_CPUCLOCK] = "hinv.cpu.clock",
    [PCP_UNAME_SYSNAME] = "kernel.uname.sysname",
    [PCP_UNAME_RELEASE] = "kernel.uname.release",
@@ -385,6 +386,7 @@ bool Platform_init(void) {
    Metric_enable(PCP_PID_MAX, true);
    Metric_enable(PCP_BOOTTIME, true);
    Metric_enable(PCP_HINV_NCPU, true);
+   Metric_enable(PCP_HINV_NDISK, true);
    Metric_enable(PCP_PERCPU_SYSTEM, true);
    Metric_enable(PCP_UNAME_SYSNAME, true);
    Metric_enable(PCP_UNAME_RELEASE, true);
@@ -753,6 +755,8 @@ bool Platform_getDiskIO(DiskIOData* data
       data->totalBytesWritten = value.ull;
    if (Metric_values(PCP_DISK_ACTIVE, &value, 1, PM_TYPE_U64) != NULL)
       data->totalMsTimeSpend = value.ull;
+   if (Metric_values(PCP_HINV_NDISK, &value, 1, PM_TYPE_U64) != NULL)
+      data->numDisks = value.ull;
    return true;
 }
 

[Explorer09]

@natoscott There is no "utilization percentage" for NetworkIOMeter at the moment. So I would consider showing the number of interfaces a feature separate from this pull request.

[natoscott]

Understood & agreed. FWIW, I think this approach is not ideal (shares the same problem of the original 100% limiting, as mentioned by the reporter of #1374 - the code was fine the way it was originally). We need to be aware here of properties of some hardware described in the iostat(1) man page ...

              %util  Percentage  of elapsed time during which I/O requests were
                     issued to the device (bandwidth utilization  for  the  de‐
                     vice).  Device  saturation occurs when this value is close
                     to 100% for devices serving requests  serially.   But  for
                     devices  serving requests in parallel, such as RAID arrays
                     and modern SSDs, this number does not reflect  their  per‐
                     formance limits.

[Explorer09]

@natoscott What was the ideal approach anyway?

[natoscott]

Just reverting to the original htop behaviour as the issue reporter suggested.

[Explorer09]

@natoscott Look in my patch carefully. I separated the utilization value into two. cached_utilisation_diff would revert to original behavior of allowing more than 100% to be shown; the bar would be drawn with cached_utilisation_norm which is the normalized value.

[Explorer09]

Should the data type be unsigned int, size_t, uint64_t or unsigned long long?
Should the property be named "numDisks" or "nDisks"?

Other than those questions, this patch is ready.

[natoscott]

@natoscott Look in my patch carefully. I separated the utilization value into two. cached_utilisation_diff would revert to original behavior of allowing more than 100% to be shown; the bar would be drawn with cached_utilisation_norm which is the normalized value.

Two thumbs up - good stuff, thanks. Regarding your data type question, I think I know what @BenBE is going to suggest: size_t FTW. :) However, systems with more than 4 billion disks are few and far between so I'm sure a 32 bit unsigned value would suffice. If noone has strong opinions, I suggest sticking with what you have now since that's consistent with the surrounding code.

[natoscott]

LGTM.

[Explorer09]

Regarding your data type question, I think I know what @BenBE is going to suggest: size_t FTW. :)

For PCP this would require a downcast from uint64_t and an (numDisks <= SIZE_MAX) check that I don't think it's worth it. That's why I chose a larger data type than size_t for now.

[natoscott]

Regarding your data type question, I think I know what @BenBE is going to suggest: size_t FTW. :)

For PCP this would require a downcast from uint64_t and...

FWIW, for PCP to use 32-bit types we would change this section:

   if (Metric_values(PCP_HINV_NDISK, &value, 1, PM_TYPE_U64) != NULL)
      data->numDisks = value.ull;

to

   if (Metric_values(PCP_HINV_NDISK, &value, 1, PM_TYPE_U32) != NULL)
      data->numDisks = value.ul;

but, keep it as you have it, it's fine IMO.