[jdk17u-dev] RFR: 8292989: Avoid dynamic memory in AsyncLogWriter [v2]

[Thomas Stuefe]

On Fri, 4 Aug 2023 19:40:10 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> This patch is *NOT* a clean backport.  The logic part is exactly the same, but I have to adjust code in the following parts:
>> 1. change from KVHashtable to ResourceHashtable because we want to use resourceArea instead of C_HEAP. 
>> 2. rewrite lambda expression using Functor. 
>> 
>> Testing: 
>> jdk-test1 tests including gtest. 
>> Manually check there's no malloc/free in runtime.
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert resourceHash.hpp and use functor instead of C++ lambda.

Hi Xin :)

> Hi, Thomas,
> 
> Thanks you for the comments. I was told that it is better reducing memory footprint of UL in jdk17u, a LTS Java.

Interesting, I'm curious about the effectiveness of your patch. Do you see good results? I would have thought that a lot of these interleaving memory requests are just taken from and handed back into fastbins or tcache, and therefore don't make the arena grow. Also, the glibc attempts to reclaim memory automatically are quite limited (see PR description https://github.com/openjdk/jdk/pull/14781#issue-1790932923)

> May I learn more about glibc autotrim? Is it a glibc feature or a new feature of hotspot?
> 
> I found [JDK-8268893](https://bugs.openjdk.org/browse/JDK-8268893) on JBS. It itself is a very useful feature! It is not an automatic trimmer, is it?

Its https://bugs.openjdk.org/browse/JDK-8293114. You have to manually enable it, it is still marked as experimental. Also not in 17 yet, but Aleksey is preparing things to get it down quickly once it has baked in head a bit.

Essentially, what the trimmer does is run periodically a malloc_trim() operation. malloc_trim() will reclaim memory that has been free'd and that is in page granularity (so, it will also reclaim memory in the middle of an arena).

Cheers, Thomas

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PR Comment: https://git.openjdk.org/jdk17u-dev/pull/1630#issuecomment-1667504164