How many threads is too many?

352
votes

I am writing a server, and I send each action of into a separate thread when the request is received. I do this because almost every request makes a database query. I am using a threadpool library to cut down on construction/destruction of threads.

My question is: what is a good cutoff point for I/O threads like these? I know it would just be a rough estimate, but are we talking hundreds? Thousands?

How would I go about figuring out what this cutoff would be?

EDIT:

Thank you all for your responses, it seems like I am just going to have to test it to find out my thread count ceiling. The question is though: how do I know I've hit that ceiling? What exactly should I measure?

12
multithreadingperformancethreadpool
@ryeguy: The entire point here is you should not be setting any maximum in the threadpool if there is no performance problems to start with. Most of the advice of limiting a threadpool to ~100 threads is ridiculous, most thread pools have /way/ more threads than that and never have a problem. – GEOCHET
ryeguy, see addition to my answer below re what to measure. – paxdiablo
Don't forget that Python is by nature, not really multi-thread friendly. At any point of time, a single bytecode opcode is being executed. This is because Python employs Global Interpreter Lock. – ASk
@Jay D: I'd say the moment you've hit the ceiling is when your performance starts to drop. – ninjalj
@GEOCHET "The entire point here is you should not be setting any maximum in the threadpool" Ummm...say what? Fixed-size thread pools have the benefits of graceful degradation and scalability. E.g. in a network setting, if you're spawning new threads based on client connections, without a fixed pool size you run the very real danger of learning (the hard way) just how many threads your server can handle, and every single connected client will suffer. A fixed-size pool acts like a pipe valve by disallowing your server from trying to bite off more than it can chew. – b1nary.atr0phy

12 Answers

244
votes

Some people would say that two threads is too many - I'm not quite in that camp :-)

Here's my advice: measure, don't guess. One suggestion is to make it configurable and initially set it to 100, then release your software to the wild and monitor what happens.

If your thread usage peaks at 3, then 100 is too much. If it remains at 100 for most of the day, bump it up to 200 and see what happens.

You could actually have your code itself monitor usage and adjust the configuration for the next time it starts but that's probably overkill.

For clarification and elaboration:

I'm not advocating rolling your own thread pooling subsystem, by all means use the one you have. But, since you were asking about a good cut-off point for threads, I assume your thread pool implementation has the ability to limit the maximum number of threads created (which is a good thing).

I've written thread and database connection pooling code and they have the following features (which I believe are essential for performance):

  • a minimum number of active threads.
  • a maximum number of threads.
  • shutting down threads that haven't been used for a while.

The first sets a baseline for minimum performance in terms of the thread pool client (this number of threads is always available for use). The second sets a restriction on resource usage by active threads. The third returns you to the baseline in quiet times so as to minimise resource use.

You need to balance the resource usage of having unused threads (A) against the resource usage of not having enough threads to do the work (B).

(A) is generally memory usage (stacks and so on) since a thread doing no work will not be using much of the CPU. (B) will generally be a delay in the processing of requests as they arrive as you need to wait for a thread to become available.

That's why you measure. As you state, the vast majority of your threads will be waiting for a response from the database so they won't be running. There are two factors that affect how many threads you should allow for.

The first is the number of DB connections available. This may be a hard limit unless you can increase it at the DBMS - I'm going to assume your DBMS can take an unlimited number of connections in this case (although you should ideally be measuring that as well).

Then, the number of threads you should have depend on your historical use. The minimum you should have running is the minimum number that you've ever had running + A%, with an absolute minimum of (for example, and make it configurable just like A) 5.

The maximum number of threads should be your historical maximum + B%.

You should also be monitoring for behaviour changes. If, for some reason, your usage goes to 100% of available for a significant time (so that it would affect the performance of clients), you should bump up the maximum allowed until it's once again B% higher.

In response to the "what exactly should I measure?" question:

What you should measure specifically is the maximum amount of threads in concurrent use (e.g., waiting on a return from the DB call) under load. Then add a safety factor of 10% for example (emphasised, since other posters seem to take my examples as fixed recommendations).

In addition, this should be done in the production environment for tuning. It's okay to get an estimate beforehand but you never know what production will throw your way (which is why all these things should be configurable at runtime). This is to catch a situation such as unexpected doubling of the client calls coming in.

43
votes

This question has been discussed quite thoroughly and I didn't get a chance to read all the responses. But here's few things to take into consideration while looking at the upper limit on number of simultaneous threads that can co-exist peacefully in a given system.

  1. Thread Stack Size : In Linux the default thread stack size is 8MB (you can use ulimit -a to find it out).
  2. Max Virtual memory that a given OS variant supports. Linux Kernel 2.4 supports a memory address space of 2 GB. with Kernel 2.6 , I a bit bigger (3GB )
  3. [1] shows the calculations for the max number of threads per given Max VM Supported. For 2.4 it turns out to be about 255 threads. for 2.6 the number is a bit larger.
  4. What kindda kernel scheduler you have . Comparing Linux 2.4 kernel scheduler with 2.6 , the later gives you a O(1) scheduling with no dependence upon number of tasks existing in a system while first one is more of a O(n). So also the SMP Capabilities of the kernel schedule also play a good role in max number of sustainable threads in a system.

Now you can tune your stack size to incorporate more threads but then you have to take into account the overheads of thread management(creation/destruction and scheduling). You can enforce CPU Affinity to a given process as well as to a given thread to tie them down to specific CPUs to avoid thread migration overheads between the CPUs and avoid cold cash issues.

Note that one can create thousands of threads at his/her wish , but when Linux runs out of VM it just randomly starts killing processes (thus threads). This is to keep the utility profile from being maxed out. (The utility function tells about system wide utility for a given amount of resources. With a constant resources in this case CPU Cycles and Memory, the utility curve flattens out with more and more number of tasks ).

I am sure windows kernel scheduler also does something of this sort to deal with over utilization of the resources

[1] http://adywicaksono.wordpress.com/2007/07/10/i-can-not-create-more-than-255-threads-on-linux-what-is-the-solutions/

19
votes

If your threads are performing any kind of resource-intensive work (CPU/Disk) then you'll rarely see benefits beyond one or two, and too many will kill performance very quickly.

The 'best-case' is that your later threads will stall while the first ones complete, or some will have low-overhead blocks on resources with low contention. Worst-case is that you start thrashing the cache/disk/network and your overall throughput drops through the floor.

A good solution is to place requests in a pool that are then dispatched to worker threads from a thread-pool (and yes, avoiding continuous thread creation/destruction is a great first step).

The number of active threads in this pool can then be tweaked and scaled based on the findings of your profiling, the hardware you are running on, and other things that may be occurring on the machine.

11
votes

One thing you should keep in mind is that python (at least the C based version) uses what's called a global interpreter lock that can have a huge impact on performance on mult-core machines.

If you really need the most out of multithreaded python, you might want to consider using Jython or something.

9
votes

As Pax rightly said, measure, don't guess. That what I did for DNSwitness and the results were suprising: the ideal number of threads was much higher than I thought, something like 15,000 threads to get the fastest results.

Of course, it depends on many things, that's why you must measure yourself.

Complete measures (in French only) in Combien de fils d'exécution ?.

6
votes

I've written a number of heavily multi-threaded apps. I generally allow the number of potential threads to be specified by a configuration file. When I've tuned for specific customers, I've set the number high enough that my utilization of the all the CPU cores was pretty high, but not so high that I ran into memory problems (these were 32-bit operating systems at the time).

Put differently, once you reach some bottleneck be it CPU, database throughput, disk throughput, etc, adding more threads won't increase the overall performance. But until you hit that point, add more threads!

Note that this assumes the system(s) in question are dedicated to your app, and you don't have to play nicely (avoid starving) other apps.

3
votes

One thing to consider is how many cores exist on the machine that will be executing the code. That represents a hard limit on how many threads can be proceeding at any given time. However, if, as in your case, threads are expected to be frequently waiting for a database to execute a query, you will probably want to tune your threads based on how many concurrent queries the database can process.

3
votes

I think this is a bit of a dodge to your question, but why not fork them into processes? My understanding of networking (from the hazy days of yore, I don't really code networks at all) was that each incoming connection can be handled as a separate process, because then if someone does something nasty in your process, it doesn't nuke the entire program.

3
votes

The "big iron" answer is generally one thread per limited resource -- processor (CPU bound), arm (I/O bound), etc -- but that only works if you can route the work to the correct thread for the resource to be accessed.

Where that's not possible, consider that you have fungible resources (CPUs) and non-fungible resources (arms). For CPUs it's not critical to assign each thread to a specific CPU (though it helps with cache management), but for arms, if you can't assign a thread to the arm, you get into queuing theory and what's optimal number to keep arms busy. Generally I'm thinking that if you can't route requests based on the arm used, then having 2-3 threads per arm is going to be about right.

A complication comes about when the unit of work passed to the thread doesn't execute a reasonably atomic unit of work. Eg, you may have the thread at one point access the disk, at another point wait on a network. This increases the number of "cracks" where additional threads can get in and do useful work, but it also increases the opportunity for additional threads to pollute each other's caches, etc, and bog the system down.

Of course, you must weigh all this against the "weight" of a thread. Unfortunately, most systems have very heavyweight threads (and what they call "lightweight threads" often aren't threads at all), so it's better to err on the low side.

What I've seen in practice is that very subtle differences can make an enormous difference in how many threads are optimal. In particular, cache issues and lock conflicts can greatly limit the amount of practical concurrency.

2
votes

ryeguy, I am currently developing a similar application and my threads number is set to 15. Unfortunately if I increase it at 20, it crashes. So, yes, I think the best way to handle this is to measure whether or not your current configuration allows more or less than a number X of threads.

-6
votes

In most cases you should allow the thread pool to handle this. If you post some code or give more details it might be easier to see if there is some reason the default behavior of the thread pool would not be best.

You can find more information on how this should work here: http://en.wikipedia.org/wiki/Thread_pool_pattern

-10
votes

As many threads as the CPU cores is what I've heard very often.