Tachyon Doesn't Seem to be Aware of Available Memory

Question

Just to see if Tachyon would give me an error about configured memory being more than available I set:

# Some value over combined available mem and disk space.
export TACHYON_WORKER_MEMORY_SIZE=1000GB

And observed the allocation in the web UI without error.

Is some of the info going to be pushed to disk when available RAM is exceeded?

What happens when it exceeds disk space? Dropped file errors or system failure?

Answer 1

This is the expected (if perhaps unhelpful behaviour) and ultimately it is to do with the fact that Tachyon uses Linux ramfs as the in-memory storage.

As this article explains:

ramfs file systems cannot be limited in size like a disk base file system which is limited by it’s capacity. ramfs will continue using memory storage until the system runs out of RAM and likely crashes or becomes unresponsive.

Note that Tachyon will enforce the size constraint based on the size you give it. However as you've found you can allocate more RAM than is actually available and Tachyon won't check this so you may want to go ahead and file a bug report.

To answer your specific questions:

• No excess data will not be pushed to disk automatically
• When RAM is full behaviour is OS dependent

Note that the setting you are referring to only controls the in-memory space, if you want to use local disks in addition to RAM then you need to use Tachyon's Tiered Storage.