About Instances and Threads, Performance and RAM consumption


Understanding how instances per core, threads per instance and ZODB caches are influencing performance.


In a usual production Zope/Plone setup there are some tunings possible. For a certain size of site, you need more than one Zope-instance and use HAproxy or Pound to load-balance between them. Then you may ask yourself: How many instances do I need? Next you see there is value “threads per instance” and wonder about the different recommendations: Only one thread or two, or four? And how does it effect memory usage?

Rule of Thumb

A good rule-of-thumb for a common setup was (and still is) two instances per core, two threads per instance, and adjust the number of objects in the ZODB cache to a number that your memory uses.

This rule may not apply when any of the following scenarios exist:

  • your setup gets more complex

  • you have several logged in users

  • you have only anonymous users

  • you use official, fancy, specific or home-grown add-ons

In any of these scenarios, you will need to figure it out yourself. It’s more important to understand the mechanism behind performance than following a set rule.

With recent, faster hardware and the (sometimes odd) behavior of virtual machines (which can be very very different dependent on the kind of VM) this needs slight or major adjustment.



A Zope instance runs a pool of threads. It queues an incoming request and dispatches it to a free thread. If no thread is free, the request remains in the queue and is dispatched when a thread is freed. If all threads are used by long-running request-to-response cycles, then this may block simple tasks such as publishing a tiny icon.

Database-Connection-Pool and its Cache:

Once a thread runs, it requests a ZODB database connection from the connection pool. It locks the connection so no other thread can use it. The connection pool opens a new connection if all existing connections are already in use. If the request-to-response cycle is finished and the thread is freed, then the connection is released back to the pool.

Memory Cache:

Each connection has its own memory cache. The file system cache is shared by all connections. Each cache can have the configured number of objects in memory. Having them in memory is important. Otherwise they are unpickled when loaded from the database, the process of which is expensive.

An instance may never get enough load so that all available threads are used concurrently. In this case you may find in the Management Interface (Zope-root -> Control_Panel -> Database -> Main DB) that there are only 2 connections, but you have 4 threads. That’s because there are never 4 connections used in parallel.

Instances and memory:

An instance creates only minimal memory usage overhead. If you have two instances, each with 2 threads, or one with 4 threads, and all threads are used in either case, it won’t make much difference (~15-20MB overhead per instance at the time of writing).

Now while Plone is running for some time, another significant (but low compared to ZODB cache) amount of RAM is consumed for RAM caching inside Zope (in other words, with plone.memoize). RAM cache is shared by all threads, but not between instances. To optimize RAM caching in a multi-instance environment, “memcached” may be used to optimize memory cache and cache usage and reduce an instances memory footprint.

In any case, most memory is used (in a common setup) to cache the ZODB.

Python GIL (global interpreter lock):

Well, yes, the GIL is mentioned here. In a threaded environment such as Zope, it has an impact on performance. But it is low, and Python has been optimized over the years. Also Zope has a lot of I/O which reduces the GIL impact. A good and important optimization is to set the right check interval for your machine. jarn.checkinterval is a good tool to test for the right value.


All theory is just theory, until it is put into practice. How does theory apply to your setup if the rule-of-thumb above does not apply?

Get measurements! First of all, you need to check what happens on your machine. Learn how to use Munin (with munin.zope), HAproxy (or Pound), or [tool of your choice here]. After that you’ll get graphs of RAM, CPU, and load, and some Zope related values. HAproxy or Pound may mark a node as “down” because all threads were blocked by long running requests. To identify these requests, collective.stats might help.

Do you have more instances or threads? This question is asked often, and cannot be answered without knowing more about the Plone scenario. We can divide it roughly into four kinds of scenarios:

  • Only or almost all users are logged in

  • Only or almost all visitors are anonymous

  • Mixed, with many users and lots of hardware resources

  • Mixed, with few users and constrained hardware resources

If you deal with logged in users, there is no easy way to cache HTML pages (highly recommended anyway for all static items) in a reverse proxy cache (i.e. Varnish) in front of Plone. Zope does much more work rendering pages. To render pages, objects need to be loaded form the database. Loading is expensive. If an object is already in the database RAM cache, then it decreases the time to render a page significantly. In a setup with lots of logged in users, we need to ensure that almost all objects are loaded already. Each thread fetches a connection from the pool, and each connection has its cache. If a user now requests a page, it is first logged in, and Zope needs a bunch of objects for this from the ZODB. Also other user specific information is loaded. Furthermore, the user may operate in an Intranet within a specific area, so these objects also need to be loaded. For example, if we now have 1 instance with 5 threads, then we have up to 5 pools (5 caches). All objects of interest are loaded 5 times in the worst case. If there’s 1 instance with 1 thread (1 cache), data is loaded only once. But if there is only one instance with one thread, then a browser shooting at the web server with lots of requests at one time fills up the request queue of the instance and may time out soon. Also a second user may want to access data at the same time, but the only thread is blocked, and the CPU idles. So the best is to stick users in a load-balancer (bind it to the __ac cookie) to 1 instance with 2 threads (also this can be adjusted dependent on your setup by testing it yourself). Provide as many instances as you can (memory consumption and CPU usage will stop you). In such a setup, usage of memcached is highly encouraged.

If you have almost all anonymous users it is much easier. You can provide fewer instances (here the rule-of-thumb of 2 instances per core applies in most cases) and increase threads. Too many threads are not good, because of the GIL. You need to find the number yourself, as it depends on hardware. Here without memcached configured, good results can be expected, because memory cache is used efficiently. Increase objects per connection cache until your memory consumption hits its hard limit, and always look at your CPU usage.

In large mixed environments with enough budget for hardware, divide your environment in two: one for logged in users, and one for anonymous users, such that the above applies.

In smaller mixed environments with less hardware resources available, you need to find your own balance. A good method is to configure your load balancer to direct logged-in users to one or two distinct instances. If there are more users, this is tricky and may take some time to figure out a good setup. As such, this is the most difficult setup.