RFR: 8304442: Defer VirtualMemoryTracker work until reporting

[Johan Sjölen]

On Sat, 18 Mar 2023 14:50:48 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> Hi,
> 
> The virtual memory tracker of NMT used to do a lot of heavy linked list calculations when memory was reserved, committed, uncommited or split up. However, the results of these calculations are actually only used when creating a native memory report. Let's not slow down the JVM unnecessarily, we can do this work at time of report instead.
> 
> In order to achieve this I've replaced the public API with a work queue:ing solution. We append each work item to a `GrowableArray` and introduce the `commit_events` method to do the actual work, which we call internally when needed.
> 
> I measured the gains in efficiency through the use of Valgrind's Cachegrind tool. I ran a `linux-x64` build with the following source code:
> 
> 
> public class Test {
>     public static void main(String[] args) {
>     }
> }
> 
> 
> These are the total cycles executed by `os::commit` and `os::reserve` as estimated by Valgrind over the entire run of the program. 
> 
> 
> os::commit
> old         | new         | new / old
> 935238      | 578979      | 0.62
> os::reserve
> old         | new         | new / old
> 53628       | 21825       | 0.41
> 
> 
> There should also be some memory savings as a `MemoryEvent` is smaller (64 bytes) than a `ReservedRegion` (96 bytes). That is, until a `commit_events()` occur.

Here are the call graphs for the new `os::commit` and the old `os::commit`. Most of the new method is inlined.

![commit_new](https://user-images.githubusercontent.com/103041674/226113355-685ed3af-2095-4c81-8e78-011afc09d8ea.png)
![commit_old](https://user-images.githubusercontent.com/103041674/226113357-c539eabd-17da-4bc3-8893-36d3c3bbc3d2.png)

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PR: https://git.openjdk.org/jdk/pull/13088