3 ms·

When the library attempts to load something from disk that doesn’t fit into memory, it’s transparently, and (usually) without extra intervention from the user,

When the library attempts to load something from disk that doesn’t fit into memory, it’s transparently, and (usually) without extra intervention from the user, swaps to memory-mapping and chunking through the file(s). Particularly useful for when you’ve got a bunch of data that doesn’t fit in memory, but setting up a whole cluster is not worth the overhead (operationally or otherwise) and/or if you’ve already got a processing pipeline written in a language/framework and you can’t/don’t-want to go through rewriting it for something distributed.

hermitcrab 6y ago
How is that different to virtual memory?
- dTal 6y ago
  Memory mapping lazily loads the file, completes immediately, and scales arbitrarily; using disk-backed virtual memory would require the entire file to be read from disk, written back out to disk, and then read in from disk again on access; it would also require swap to be set up at the OS level, and the amount of swap set up puts a hard limit on the size of the file.
  - wtallis 6y ago
    You've described the difference between using mmap vs relying on the operating system's swap mechanism. But neither of those is quite the same as having an application that's aware of its memory usage and explicitly manages what it keeps in RAM. Using mmap may be useful for achieving that, but mmap on its own still leaves most of the management up to the OS.
  - hermitcrab 6y ago
    Thanks for clearing that up!
- nijave 6y ago
  In memory representation also tends to have some data structure overhead so 5GB compressed -> 50GB uncompressed -> 100-250GB as a data structure (seems 2-5x is pretty normal). It starts out seeming pretty innocuous but quickly explodes. In addition, some things like Drill can do some automatic indexing/metadata recording which can reduce the amount of data it needs to access

hermitcrab 6y ago

How is that different to virtual memory?

dTal 6y ago
Memory mapping lazily loads the file, completes immediately, and scales arbitrarily; using disk-backed virtual memory would require the entire file to be read from disk, written back out to disk, and then read in from disk again on access; it would also require swap to be set up at the OS level, and the amount of swap set up puts a hard limit on the size of the file.
- wtallis 6y ago
  You've described the difference between using mmap vs relying on the operating system's swap mechanism. But neither of those is quite the same as having an application that's aware of its memory usage and explicitly manages what it keeps in RAM. Using mmap may be useful for achieving that, but mmap on its own still leaves most of the management up to the OS.
- hermitcrab 6y ago
  Thanks for clearing that up!
nijave 6y ago
In memory representation also tends to have some data structure overhead so 5GB compressed -> 50GB uncompressed -> 100-250GB as a data structure (seems 2-5x is pretty normal). It starts out seeming pretty innocuous but quickly explodes. In addition, some things like Drill can do some automatic indexing/metadata recording which can reduce the amount of data it needs to access