Redis - Default blocking VM

Question

The blocking VM performance is better overall, as there is no time lost in synchronization, spawning of threads, and resuming blocked clients waiting for values. So if you are willing to accept an higher latency from time to time, blocking VM can be a good pick. Especially if swapping happens rarely and most of your often accessed data happens to fit in your memory.

This is default mode of Redis (and the only mode going forward I believe now VM is deprecated in 2.6), leaving the OS to handle paging (if/when required). I am correct in my understanding that it will take some time to get "hot" when booted/started. When working on a 1gb RAM node with a 16gb dataset, does Redis attempt to load it all into virtual memory at boot and thus 90%+ is immediately paged out, and only after some good amount of usages does the above statement hold true?

Answer 1

Redis VM was already deprecated in Redis 2.4, and has been removed in Redis 2.6. It is a dead end: don't use it.

I think you are confusing the blocking VM with OS paging. They are two different things.

OS paging is the default mode of Redis when Redis VM is not configured at all (whatever the blocking mode). The OS will swap Redis memory if it does not fit in physical memory. The event loop can be frozen at any time. When it happens, performance is abysmal because none of the Redis internal data structures is designed for this (no locality, no paging system).

Redis VM can be configured in non blocking mode (using I/O threads). When I/Os are done, the event loop is not blocked, and Redis is still responsive. However, when too many I/Os pile up, the I/O threads will be completely busy, and you end up with a responsive Redis, but unable to process any queries requiring I/Os.

Redis VM can also be configured in blocking mode. In this mode all I/Os are synchronously performed in the main event loop thread. So the event loop is frozen in case of I/O (for instance in case of a key miss). All clients are impacted. However, general performance (CPU consumption and latency) is better than with the non blocking mode because some threading scheduling/synchronization is saved.

In practice, the difference between OS paging and the Redis blocking VM is the granularity level. With Redis VM, the granularity is the key. With OS paging, well it is the page (a 4 KB block which can span on several unrelated keys).

In all 3 cases, the initial load of the dump file will be extremely slow and generate a peak of random I/Os on your system. As you pointed out, most objects will be loaded and then swapped out. The warm-up time will be significant.

Except if you have extreme locality in your data, or if you do not care at all about the latencies, using 1 GB RAM for a 16 GB dataset with the Redis VM is science-fiction IMO.

There is a reason why the Redis VM was phased out. By design, it will never perform as well as a disk-based datastore (which can exploit file mapping or direct I/Os to avoid the double buffering, and use adapted data structures like B-trees).

Redis as an in-memory store is excellent. But if you need to store something which is bigger than RAM, don't use it. Other (disk-based) stores will all perform much better.