Allow to specify ratio of allocation of available memory between heap and direct memory

[mmanciop]

Applications that heavily use NIO, like some Netty-based ones, sometimes benefit from more direct memory as the overall memory available to the JVM increases. This is because direct memory does not have, in itself, garbage collection, but rather it relies on APIs like Cleaner or other phantom reference-based ones to free direct memory as a consequence of evicting objects from heap (like ByteBuffers).

We actually have one such use-case in the Instana Agent, which is a Netty-based application that uses significantly more direct memory than most Java apps. When the Instana Agent runs in a container, we usually want to allocate roughly 1MB to direct memory for each 3MB to the heap, although heavy usage of some functionality skews that ratio one way or the other.

We have forked the memory calculator for tweaking it to our needs, but we'd be happy to contribute back if this feature is interesting to this project.

[dmikusa]

Thanks for reaching out!

I have not worked with very many users that need to customize the direct memory size. The default seems to work for many. Of those that I have seen customize it, it's usually something trivial like go up to 15 or 20M till you stop getting OOMEs. I don't think I've seen a relationship between heap & direct memory size like you're seeing in your code.

I would suggest that we leave this issue open. If anyone is interested in this feature, give it a 👍 and we can talk about integrating it.

[mayrstefan]

I would say this is instana@576faff and instana@adad7e4

As we see more applications make use of DirectMemory (mostly using Netty) this feature is becoming more interessting.

[postalservice14]

@mmanciop Is it an Instana specific requirement that needs the ratio to heap? Or is that Netty? I'm curious because lettuce-io uses netty and we've been running into OOMEs. I plan to just increase the direct memory size, but after seeing this, was curious if it's a netty thing that uses the ratio.

[mmanciop]

@postalservice14 I did this contribution to the buildpack before I joined Instana. Netty was the classic kind of library that needed more NIO direct memory. But IIRC, it’s not necessary just Netty: you may have multiple libraries using NIO and needing direct memory.

P.S. I left Instana years ago, so I won’t comment on whether their proprietary Java tracing uses NIO or not behind the scenes.

[postalservice14]

@mmanciop thanks - yeah, I follow all that. I just wasn't sure if you discovered that working with netty itself is what needed the ratio or if the reasoning was more on the instanta side. Because I agree with the other commenters that a ratio does seem odd to me. Thanks for following up!

[mmanciop]

In the scope of how the memory calculator works, the ratio made sense to me at the time: for the memory “allocatable from the logic” (heap + direct memory), specify the ratio between them.