Multi-thread read performance

[olsonanl]

I've been experimenting with NuDB and ran into the following issue - I would like to have multiple worker threads access the database concurrently, entirely read-only. There seems to be a bottleneck where I only get 2-3x speedups; intel profiler showed what looks to be locking overhead. Should I be able to see higher concurrency for this? Or perhaps am I seeing I/O read rate issues (the data file is in /dev/shm ramdisk for this testing)

Thanks,
Bob

[vinniefalco]

yes you should be seeing higher concurrency if there are more than 3 threads.

[olsonanl]

OK, thanks. I'll do some more digging, including putting the data back on the SSD volume. Do you think it's possible the performance characteristics of the ramdisk may be at fault?

[olsonanl]

I'm seeing a speedup of about 3.7 on 6 threads. The serial version of the code is seeing about 60% system CPU; I wonder if I'm just hitting disk overheads. Data records are small; 8 byte keys and 10 byte records. 600M entries.

(Prior version of this code loaded up an in-memory hashtable from a memory map; trying to get the app working with as little startup overhead as possible without the current solution of requiring either vmtouch-locked data or persistent server)

[vinniefalco]

Do you think it's possible the performance characteristics of the ramdisk may be at fault?

I don't know

[reder2000]

Is NuDB multiprocess ? For example, RocksDB is not very good at it.

[vinniefalco]

NuDB is not multi-process. You embed it in your own process, like sqlite.

[reder2000]

I would like to build something like diskcache (python) for c++. They use SQLite and have no problems with multiprocess (I have not looked why I confess). I have not measured performance since it is python anyway...
But me using SQLite C++ performance quickly degraded. Rocksdb was solid but monoprocess. Postgresql could be taped multiprocess with mixed performance.
I consider building a IPC memory sharing server with maybe nudb as backbone....
Thanks for the answer and your precious work.

[vinniefalco]

You could build a socket-based server on top of NuDB and use it that way