On Monday, 12 April 2021 at 07:03:02 UTC, Sebastiaan Koppe wrote:

We have similar problems, we see memory usage alternate between plateauing and then slowly growing. Until it hits the configured maximum memory for that job and the orchestrator kills it (we run multiple instances and have good failover).

I have reduced the problem by refactoring some of our gc usage, but the problem still persists.

On side-note, it would also be good if the GC can be aware of the max memory it is allotted so that it knows it needs to do more aggressive collections when nearing it.

I knew this must be a more common problem :)

What I've found in the meantime:

* nice writeup of how GC actually works by Vladimir Panteleev - https://thecybershadow.net/d/Memory_Management_in_the_D_Programming_Language.pdf * described tool (https://github.com/CyberShadow/Diamond) would be very helpfull, but I assume it's for D1 and based on some old druntime fork :( * we've implemented log rotation using `std.zlib` (by just `foreach (chunk; fin.byChunk(4096).map!(x => c.compress(x))) fout.rawWrite(chunk);`) * oh boy, don't use `std.zlib.Compress` that way, it allocates each chunk and for a large files it creates large GC memory peaks that sometimes doesn't go down
  * rewritten using direct `etc.c.zlib` completely out of GC
* currently testing with `--DRT-gcopt=incPoolSize:0` as otherwise allocated page size multiplies with number of allocated pools * 3MB by default * `profile-gc` is not much helpfull in this case as it only prints total allocated memory for each allocation on the application exit and as it's a long running service using many various libraries it's just hundreds of lines :) * I've considered to fork the process periodically, terminate it and rename the created profile statistics to at least see the differences between the states, but still not sure if it would help much * as I understand the GC it uses different algorithm for small allocations and for large objects
  * small (`<=2048`)
* it categorizes objects to fixed set of used sizes and for each uses whole memory page as bucket with free list from which it reserves memory on request * when the bucket is full, new page is allocated and allocations are provided from that
  * big - similar, but it allocates N pages as a pool

So If I understand it correctly when for example vibe-d initializes new fiber on some request, it's handled and fiber can be discarded it can easily lead to a scenario when fiber itself is allocated in one page, it's filled up during the request processing so new page is allocated and when cleaning, bucket with fiber cannot be cleaned up as it's added to a `TaskFiber` pool (with a fixed size). This way fiber's bucket would never be freed and easily never be used anymore during the application lifetime.

I'm not so sure if pages of small objects (or large) that are not completely empty can be reused as a new bucket or only free pages can be reused.

Does anyone has some insight of this?

Some kind of GC memory dump and analyzer tool as mentioned `Diamond` would be of tremendous help to diagnose this..

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