Excel: How to Adjust the Scale of the Axis in a Scatter Chart

Routinely, we have been using TextPad and Excel to analyze Garbage Collection (GC) events gathered in GC log files.

In this article, we will demonstrate how to use these data to create a Scatter Chart and also adjust the scale of the horizontal axis in the chart as shown below:

Before Scale Change

After Scale Change

The Data

After cleaning the data, we have two columns holding data as shown below:

• GC time stamp
• Mixed GC pause time

Using shift key, you can select both A1 and B1 cells as shown below:

Then you can select entire columns by pressing:^[1]

CTRL+SHIFT+DOWN ARROW key

Create a Scatter Chart from Spreadsheet Data

A scatter chart has two value axes, showing one set of numerical data along the x-axis and another along the y-axis. It combines these values into single data points and displays them in uneven intervals, or clusters.

To create a scatter chart, you do:^[2]

1. Arrange your data so that the x-values are in the first column of your worksheet, and the y-values are located in adjacent columns.
2. Select the range of x- and y-values that you want to plot in the chart.
3. Click Chart on the Insert menu to start the Chart Wizard.
4. In the Chart type box, select Scatter .
5. Under Chart sub-type, click the chart sub-type you want to use.

This will create a scatter chart as shown in diagram 1.  But, you can see the scale of X-axis is not appropriate.

Change the Scale of Horizontal Axis

By default, Microsoft Office Excel determines the minimum and maximum scale values of the axis when you create a chart. However, you can customize the scale to better meet your needs.  To customize the scale of horizontal axis, you do:

1. Change the label/title of the series to "Mixed GC"^[4]
2. In the chart, click the horizontal (value) axis that you want to change.
3. This displays the Chart Tools, adding the Design, Layout, and Format tabs.
4. On the Format tab, in the Current Selection group, click the arrow next to the Chart Elements box, and then click Horizontal (Value) Axis.
5. In the Current Selection group, click Format Selection.
6. In the Format dialog box, change maximum value of X-axis from 40000.0 to 30000.0.

Voila, you have created a scatter chart of the Mixed GC diagram with a good scale at horizontal axis (see diagram 2 above).

References

1. Select cells, ranges, rows, or columns on a worksheet
2. Creating XY (Scatter) and Line charts
3. Change the scale of the vertical (value) axis in a chart
4. Edit titles or data labels in a chart

HotSpot: A Case Study of MetaspaceSize Tuning in JDK 8

This article is one of the Metaspace series in JDK 8 on Xml and More.  Please read previous articles (i.e. [1, 2, 3]) for the background information.

Here we use a case study to show you what we mean by:

Proper monitoring and tuning of the Metaspace is still required in order to limit the frequency or delay the garbage collections of metaspace.

 

JVM Setup

We have used the following print options:

•  -XX:+PrintGCDetails -XX:+PrintGCTimeStamps

to generate GC log files.  Besides that, we have set:

•  -XX:MetaspaceSize=128m

to specify the initial high water mark to trigger metaspace cleanup by inducing a GC event.

In the GC log file, you can find the following entries:

[Metaspace: 21013K->21013K(1069056K)]

The first value shows you the previously used size before GC, the second value shows you the current used size and the last value shows you the current reserved size.

Before Tuning

Below is the experiment running with all default values.  As you can see, there are six Full GC events triggered by "Metadata GC Threshold" being reached.  The initial high water mark was set by the default MetaspaceSize (i.e.,  21807104 Bytes).  So, the first Full GC event was induced when committed memory of all metaspaces reaches the initial high water mark.  To find HotSpot's default option values, read [4].

3.112: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 21013K->21013K(1069056K)]
5.440: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 34645K->34645K(1081344K)]
11.921: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 58159K->58152K(1101824K)]
18.321: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 97038K->97038K(1136640K)]
51.761: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 160475K->155432K(1193984K)]
319.406: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 267854K->267854K(1288192K)]

Note that the unit on X-axis is seconds and Y-axis KBytes.

After Tuning

Here we have set the experiment with the following extra option:

• -XX:MetaspaceSize=128m

Note that 128M is bigger than default value (i.e., 21807104 Bytes).  Because of the setting, we have reduced the frequency and delayed garbage collections due to "Metadata GC Threshold" being reached.  

25.863: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 127978K->126460K(1165312K)]
81.254: [Full GC (Metadata GC Threshold) ...]
   [Eden: ..., [Metaspace: 213481K->209918K(1241088K)]
3486.808: [Full GC (Allocation Failure) ...]
   [Eden: ..., [Metaspace: 306929K->298136K(1327104K)]
3631.001: [Full GC (Allocation Failure) ...]
   [Eden: ..., [Metaspace: 299979K->298792K(1329152K)]

Why to Tune?

So, you may ask what's the deal with tuning MetaspaceSize.  Yes, it's just setting the initial high water mark.  Later HotSpot will adjust high water mark based on ergonomics.  Hopefully, when HotSpot's ergonomics becomes more matured, no tuning is necessary at all.  But, even in that case, you may still want to set MetaspaceSize for some occasions.  One of them is when you run a benchmark in a short period and your focus may be on other GC activities than meta data being loaded or unloaded.

References

1. HotSpot: Understanding Metaspace in JDK 8
2. HotSpot: Monitoring and Tuning Metaspace in JDK 8
3. jstat Tool: New Metaspace Statistics from -gc Option
4. What Are the Default HotSpot JVM Values?
5. VM Class Loading