[BUG] KeyDB w/ flash hang during aggressive and blocking evictions

[gloomy-7164]

Describe the bug

KeyDB hang and was not responsive for a few seconds when it enabled flash and memory utilization grew to reach maxmemory.

It hang for a few seconds, client read/write were almost blocked and saw 1000+ ms latency. KeyDB flushed multi hundreds of MBs of data at once. maxmemory_policy = allkeys_random

I was testing keydb 6.3.3, on AWS r7gd.xlarge instance, 1 primary 0 replica.

To reproduce

1. Spin up a keydb cluster with flash enabled.
2. Write data into the cluster until memory utilization reaches maxmemory limit.
3. KeyDB will enter evictions and spend several seconds busy evicting and almost not responsive. redis-cli commands took seconds to return, client throughput dropped to almost zero.

Expected behavior Evictions should happen real-time and incrementally and keydb should not spend much time evicting hundreds of MBs of data in a blocking way.

Additional information

I think the problem is from 2 places in the code:

1. KeyDB decides to free 5% of memory at once if storage is enabled. Code pointer, and it was from this commit. It means if maxmemory=10G, keydb will evict 500MB of data all in once which takes 5+ seconds to finish. @JohnSully Any reasons why we want to evict aggressively if storage is enabled? If evictions only involves removing data from memory and not deleting them from rocksdb, I think it should be ok to evict in the same way as non-storage mode?
2. Redis has a mechanism to limit the max time spent in a single eviction attempt, controlled by the maxmemory-eviction-tenacity config. However this exit-early mechanism is disabled when storage is enabled, see code. What will be bad if we chose to exit evictions earlier and have storage enabled?

[keithchew]

Hi

I encountered this issue a while back:

https://github.com/Snapchat/KeyDB/issues/748

Workaround can be found in the link above to keep the eviction routine in the microseconds range while keeping the CPU usage tamed.

[gloomy-7164]

Hi

I encountered this issue a while back:

748

Workaround can be found in the link above to keep the eviction routine in the microseconds range while keeping the CPU usage tamed.

Thanks Keith. I think our issues were different, as in my case there were no expires involved, but only the maxmemory related evictions. And my flamegraph showed very little overhead with rocksdb but mostly in the performEvictions logic itself.

(Note that this flamegraph was obtained with allkeys-lru not allkeys-random mentioned in the original issue )

[gloomy-7164]

FYI I made a simple change here to re-enable the early exit in evictions, and the blocking evictions were almost gone.

if ( /* g_pserver->m_pstorageFactory == nullptr &&  COMMENT OUT THE STORAGE CHECK*/ elapsedUs(evictionTimer) > eviction_time_limit_us) {

Keydb was responsive during evictions, max latency dropped from 1000+ms to ~40 ms.

[keithchew]

Hi @gloomy-7164

Ah, I just realised your issue is in evict.cpp, while mine was in expire.cpp, apologies for not picking that up earlier! I have not experienced any issues on evictions because I have not stress tested maxmemory yet (that test is coming up, currently stress testing replication). Your patch looks handy, so will also try it when I get to that stage.