[Unison-hackers] Memory exhaustion issue (#1068)

[Michael von Glasow]

Hi all,

I am experiencing memory issues after upgrading the entire OS on my file
server, and going from Unison 2.52.1 (built by myself) to 2.53.3-2build2
(from the Ubuntu repos).

The server is a Raspberry Pi 3 with 1 GByte of physical memory, as well
as 3 GB swap space on a USB disk. It previously ran Raspbian stretch
(32-bit), now Ubuntu 24.04 (noble, 64-bit).

I was previously able to transfer 16 Gbyte files (possibly even multiple
ones) using Unison over SSH with no issues. I can also copy files of
that size using SSH or CIFS, with data transfer rates in the range of
5–7.5 Mbyte/s (over a 100 Mbit Ethernet connection).

Now, when transferring a 16 GByte file, the first 1% (as indicated by
the GUI on the client side, also 2.53.3) transfer es expected, at about
5 MByte/s. However, I notice memory usage increasing, with Unison
occupying just above 60% of physical memory.

Having reached about 1%, total real memory usage on the system goes to
just above 80%, while virtual memory has climbed to 20–25%. At that
point, the system becomes unusable as it is just busy swapping.

Please let me know what kind of information you would need from me so
you can investigate this issue further.

Michael