RFR: 7885: Graphical rendering of dependency view fails due to heap memory drain [v2]

[Vincent Alexander Beelte]

On Fri, 4 Aug 2023 15:25:11 GMT, Virag Purnam <vpurnam at openjdk.org> wrote:

>> When multiple views are enabled in JMC mainly Dependency View and Heatmap View. (Issue was with Flame graph as well but it got fixed as the implementation is now based on swing component)
>> 
>> For the Dependency View and Heatmap View still the implementation is based on javascript and for one particular scenario JMC gives "**java.lang.OutOfMemoryError: Java heap space**". 
>> 
>> **Scenario:**
>> For each selection in a table, views get rendered. For each click on table, 4 threads call method "**toJsonString(IItemCollection items)**" present in "**IItemCollectionJsonSerializer**" class. Within method it appends the items to a StringBuilder and then it writes it to a StringWriter. When we have multiple JFR files open in editor, or we select the table contents very fast, multiple threads call the method at the same time and all try to append and write at the same time. This results in the "**java.lang.OutOfMemoryError: Java heap space**". 
>> 
>> ![image](https://github.com/openjdk/jmc/assets/97600378/ae24614c-c640-4dc0-9c4c-7f70ee2f164f)
>> 
>> ![image](https://github.com/openjdk/jmc/assets/97600378/a41434d7-7bb1-47a0-bb7f-6a8b8e17af30)
>> 
>> 
>> **Possible solution:** Making method  "**toJsonString(IItemCollection items)**" synchronized. I have done the changes and created a PR. 
>> 
>> Could you please review and provide your comments on this? If there are better way to solve this issue, could you please suggest me the same?
>
> Virag Purnam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   7885: Graphical rendering of dependency view fails due to heap memory drain

I also made an attempt to work around this problem with the idea of adding stop flags like I did in #502 . You can find the results over in https://github.com/grandmasterpixel/jmc/tree/more_stop_flags if you are interested (I also did some experiments with making the generation of the graph view go faster in there. Just ignore that).
In contrast to making json generation mutually exclusive that would reduce concurrent generation of JSONs by stopping unneeded computations as early as possible. So you do not use heap for JSONs that are thrown away soon anyway.
I never made a pull request out of that because I think the real problem is having to use JSONs at all. I found it pretty easy to get OOMEs not because of to heap exhaustion but because of array size limits anyways.

-------------

PR Comment: https://git.openjdk.org/jmc/pull/511#issuecomment-1679990275