Clever use of persistent_term

September 09, 2019 · by Lukas Larsson .

This blog post will go through three different uses of persistent_term that I have used since its release and explain a bit why they work so well with persistent_term.

Global counters

Let’s say you want to have some global counters in your system. For example the number of times an http request has been made. If the system is very busy that counter will be incremented many many times per second by many different processes. Before OTP-22 the best way that I know of to get the best performance is by using a striped ets tables. i.e. something like the code below:

incr(Counter) ->
  ets:update_counter(?MODULE,{Counter,erlang:system_info(scheduler_id)},1).

read(Counter) ->
  lists:sum(ets:select(?MODULE,[{{{Counter,'_'},'$1'},[],['$1']}])).

The code above would make sure that there is very little contention on the ets table as each scheduler will get a separate slot in the table to update. This comes at the cost of more memory usage and that when reading the value you may not get an exact value.

In OTP-22 the same can be achieved by using counters. Counters have built-in support for striping by using the write_concurrency option, so we don’t have to write our own implementation for that. They are also faster and use less memory than ets tables, so lots of wins.

The remaining problem then is finding the reference to the counter. We could put it into ets and then do an ets:lookup_element/3 when updating a counter.

cnt_incr(Counter) ->
    counters:add(ets:lookup_element(?MODULE,Counter,2),1,1).

cnt_read(Counter) ->
    counters:get(ets:lookup_element(?MODULE,Counter,2),1).

This gives a performance degradation of about 20%, so not really what we want. However, if we place the counter in persistent_term like the code below we get a performance increase by about 140%, which is much more in line with what we wanted.

cnt_pt_incr(Counter) ->
    counters:add(persistent_term:get({?MODULE,Counter}),1,1).

cnt_pt_read(Counter) ->
    counters:get(persistent_term:get({?MODULE,Counter}),1).

The reason for this huge difference is because when the counters are placed into persistent_term they are placed there as literals which means that at each increment we not longer have to make a copy of the counters reference. This is good for two reasons:

1) The amount of garbage will decrease. In my benchmarks the amount of garbage generated by cnt_incr is 6 words while both ets_incr and cnt_pt_incr create 3 words.

2) No reference counts have to be modified. What I mean by this is that the counters reference is what is called a magic reference or nif resource. These references work much in the same way as reference counted binaries in that they are not copied when sent to different processes. Instead only a reference count is incremented at copy and then decremented later by the GC. This means that for cnt_incr we actually have 3 counters that are modified for each call. First we increment the reference count on the counter when copying from ets, then we update the actual counter and then eventually we decrement the reference counter. If we use persistent_term, the term is never copied so we don’t have to update any reference counters, instead we just have to update the actual counter.

However, placing the counter in persistent_term is not trouble free. In order to delete or replace the counter reference in persistent_term we have to do a global GC which depending on the system could be very very expensive.

So this method is best to only be used by global persistent counters that will never be deleted.

You can find the code for all the above examples and the benchmark I ran here.

Logger level check

In logger there is a primary logging level that is the first test to be done for each potential log message to be generated. This check can be done many times per second and needs to be very quick. At the moment of writing (OTP-22) logger uses an ets table to keep all its configuration which includes the primary logging level.

This is not really ideal as doing a lookup from the ets table means that we have to take a read-lock to protect against parallel writes to the value. Taking such a read lock is not terribly expensive, but when done thousands of times per second it adds up.

So in this PR I’ve used persistent_term as a cache for the primary logging level. Now when reading the value from the hot path logger will instead use persistent_term. This removes all locks from the hot path and we only need to do a lookup in the persistent_term hash table.

But what if we need to update the primary logger level? Don’t we force a global GC then? No, because the small integer representing the primary logger level is an immediate. This means that the value fits in one machine word and is always copied in its entirety to the calling process. Which in turn means that we don’t have to do a global GC when replacing the value.

When doing this we have to be very careful so that the value does not become a heap value as the cost of doing an update would explode. However, it works great for logger and has reduced the overhead of a ?LOG_INFO call by about 65% when no logging should be done.

Large constant data

We use an internal tool here at the OTP-team called the “ticket tool”. It basically manages all of the OTP-XYZ tickets that you see in the release notes that comes with each release of Erlang/OTP. It is an ancient tool from late 90’s or early 00’s that no one really wants to touch.

One part of it is a server that contains a cache of all the 17000 or so tickets that have been created through the years. In that server there is a single process that has each ticket and its state in order to speed up searching in the tickets. The state of this process is quite large and when it is doing a GC it takes somewhere around 10 seconds for it to finish. This means that about every 10 minutes the server freezes for 10 seconds and we get to experience the joy of being Java programmers for a while.

Being a VM developer I’ve always thought the solution to this problem is to implement either an incremental GC or at least a mark and sweep GC for large heaps. However, the ticket tool server has never been of high enough priority to make me spend a year or two rewriting the GC.

So, two weeks ago I decided to take a look and instead I used persistent_term to move the data from the heap into the literal area instead. This was possible to do because I know that the majority of tickets are only searched and never changed, so they will remain in the literal area forever, while the tickets that do get edited move onto the heap of the ticket server. Basically my code change was this:

handle_info(timeout, State) ->
  persistent_term:put(?MODULE,State),
  erlang:start_timer(60 * 60 * 1000, self(), timeout),
  {noreply,persistent_term:get(?MODULE)}.

This small change puts the entire gen_server state into the literal area and then any changes done to it will pull the data into the heap. This dropped the GC pauses down to be non-noticeable and took considerable less time to implement than a new GC algorithm.