lock free database

While discussing some ideas with the owner of liblfds I was thinking about some of the issues in the database of Directory Server, and other ldap products. What slows us down?

Why are locks slow?

It’s a good idea to read this article to understand Memory Barriers in a cpu.

When you think about the way a mutex has to work, it takes advantage of these primitives to create a full barrier, and do the compare and exchange to set the value of the lock, and to guarantee the other memory is synced to our core. This is pretty full on for cpu time, and in reverse, to unlock you have to basically do the same again. That’s a lot of operations! (NOTE: You do a load barrier on the way in to the lock, and a store barrier on the unlock. The end result is the full barrier over the set of operations as a whole.)

Lock contenion is really the killer though. If every thread is blocked on a single lock, they cannot progress. Given the cost of our lock, if we are stalling our threads behind a lock, we have cpus waiting to do nothing during this process. The OS scheduler helps mask this by waking and running another thread, but eventually contenion will win out.

If we bring NUMA into the picture, our mutex may be in a different NUMA region than the thread requesting the lock. This adds an impact to latency as well!

We need to try and avoid these operations if we can, to increase performance.

BDB

Currently, BDB basically serialises all operations over a set of locks to access to the data.

This means that a set of readers and writers will execute in order, with only one at a time.

Not so great for performance, but great for consistency. We are hit hard by our locks, and we have issues with NUMA, especially accessing the page caches, as we regularly have to cross NUMA regions to access the required memory.

LMDB

LMDB does somewhat better. This is based on a COW btree, with reference counting to accessors. There are still locks scattered through out the tree, which will have an impact however.

But, LMDB can establish a read only transaction, of which there can be many, and a serialised, single write transaction. These still suffer from synchronisation of the locks across cores, because LMDB basically allows direct memory access to the tree.

As well, NUMA is an issue again: Across a NUMA region, if you access the DB over one of the boundaries, you will suffer a large performance hit. LMDB tries to offset this through it’s use of the VFS for page cache. However that’s just passing the buck. You now rely on the kernel to have the memory pages needed in the correct region, and that’s not guaranteed.

What can we do?

We need to think about how CPU and RAM works. We need to avoid crossing NUMA regions, and we need to avoid costly CPU instructions and behaviour. We need to avoid contenion on single locks no matter where they may be. We need our program to act less like a human reasons about it, and more like how a CPU works: Asynchronously, and with interprocessor communication. When our program behaviour matches the hardware, we can achieve better performance, and correctness.

In the testing of lfds, the lfds author has found that Single Thread, accessing memory within one NUMA region, and without locks, always wins by operation count. This is compared to even lock free behaviours across many cores.

This is because in a single thread we don’t need to lock data. It has exclusive access, and does not need to contend with other cores. No mutexes needed, no barriers needed.

So we must minimise our interprocessor traversal if we can, but we want to keep data into a single CPU region. Our data should ideally be in the NUMA region we want to access it in, in the end.

Async db

Disclaimer: This is just an idea, and still needs polish.

We run our database, (be it lmdb, bdb, or something new) in a single thread, on one cpu. Now that we are within a single CPU, we can dispense all locking mechanisms, and still have a guarantee that the view of the data is correct.

Every thread of our application would then be “pinned” to a NUMA region and core, to ensure that they don’t move.

We would then use the single producer, single consumer bounded queue from this article. Each NUMA region would establish one of these queues to the database thread. The bounding size is the number of working threads on the system. Each queue would be thread max for the bound, even though there are multiple regions. This is because there may be an unequal distribution of threads to regions, so we may have all threads in one region.

Now, our database thread can essentially round robbin, and dequeue requests as they enter. We can use the DB without locks, because we are serialised within one thread. The results would then be placed back into the queue, and the requestor of the operation would be able to examine the results. Because we put the results into the memory space of the requestor, we pay the NUMA price once as we retrieve the result, rather than repeatedly like we do now where we access various caches and allocations.

Why would this be better?

  • We have dispensed completely with ALL mutexes and locks. The queue in liblfds is fast. Amazingly fast. Seriously, look at these benchmarks. And that’s on the many many queue, which is theoretically slower than the single single bounded queue.
  • We keep consistency within the database. Because we only have one thread acting on the data, we have gained serialisation implicitly.
  • We keep data in the NUMA regions where it needs to be. Rather than having a large cache that spans potentially many NUMA regions, we get data as we need, and put it into the numa region of the DB thread.
  • Because the data is within a single thread, we take advantage of the cpu cache heavily, without the expense of the cpu caches to the other threads. Minimising page evictions and inclusions is a good thing.

There are many other potential ways to improve this:

  • We could potentially cache entries into the NUMA region. When a search is requested, provided the serial of the entry hasn’t been advanced, the entry still within our NUMA region is valid. This prevents more moving between NUMA regions, again yielding a benefit. It would basically be:
    Thread A                              Thread DB
Queue a read transaction with
    thread ID X               -->
                              <--  Open the transaction, and stash

Queue a search for set of IDs -->
                                    Dequeue search request
                                    Build ID set
                                    With each ID, pair to the "db version" of the entry. IE USN style.
                              <--   Return ID set to queue
Examine the local cache.
if ID not in local cache ||
    not USN matches entry in cache
        Add Id to "retrieve set"
Queue a retrieve request      -->
                                    Dequeue the retrieve request
                                    Copy the requested IDs / entries to Thread A
                              <--   Return

Queue a transaction complete  -->
                              <--   Done

Acknowledgement

Huge thank you to Winterflaw, the author of LibLFDS for discussing these ideas, his future review of this idea, and for teaching me many of these concepts.