How to compare verbose garbage collection from two different runs using GCMV
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Garbage Collection and Memory Visualizer (GCMV) is a great tool to visualize verbosegc and it is available for free in the IBM Support Assistant. One of its lesser well known features is the ability to compare verbosegc from two different JVM runs. This is particularly useful if you changed something and you want to see the effect.
First, load the baseline verbosegc as normal. Next, right click anywhere in the plot area and click 'Compare File...':
Next, ensure that the X-axis uses a relative format such as hours, instead of date. Otherwise, you'll just end up essentially combining the two verbosegc files with a date gap in between and so you won't be able to visualize any differences.
Finally, zoom in to the part where they overlap (i.e. one might be longer than another, so cut the extra off). Important note: GCMV's zoom feature is only a different visualization of the line plot -- it does not affect GCMV's report tab. That means that if something went wrong outside your zoom which you don't care about, zooming in to avoid that section will not disregard that errant data in the report tab (for things such as proportion of time spent in GC, largest allocation, etc.). To do this, you'll also want to change the Minimum and Maximum X values in the Axes view to approximately match your zoom. It is easiest to first change the X-axis, at which point GCMV will gray out the disregarded data. Then, you can zoom around the non-disregarded data using your cursor.
For each series, there will be a solid line for the baseline verbosegc and a dashed line of the same color for the compared verbosegc. When you click on the report tab, GCMV will create a column for each verbosegc for easier comparison:
In this case, we can see that, for example, the proportion of time spent in GC went from 7.82% to 1.48% (the native_stderr.log was the newer one). Many of the other statistics got better. In this case, we can say that the tuning we did (increasing the nursery size) was very beneficial, all other things being equal.
Now one very important consideration is "all other things being equal." You have to be very careful comparing verbosegc. If, for example, a different amount or rate of work came into these independent runs (for example, a different test was run, or one day was a workday and another a weekend with less work, etc.), then it would be much more difficult to conclude anything. One obvious sign of this is that you're tuning something like the nursery, and the overall Java heap usage is magnitudes different. The point is: carefully control your experiment to hold all other variables constant (and verify using data such as request count, response times, etc.).