arpan

Genserver performance - what would be best for this scenario?

Hi Everyone

I am making a multiplayer game and that uses phoenix as the backend. Its heavily dependent on Phoenix channels, there is no database all data is stored in ETS.

Now, currently, I have used a Genserver backed ETS store as shown here.

From what I read ETS is very fast and should not be a bottleneck, however, I also understand that genservers process messages sequentially.

Now all the game state is stored in a single ETS table. There is a genserver GameStore which provides a public API to perform operations on the single-game ETS table.
All game data will be in the same table with the game id as the key and a struct as the value.

When the number of users on the website increases there could be a case when say 100 games are played at once, and every game will frequently change its state which requires updating or reading from the ETS table by making calls to the GameStore genserver.

I am wondering how this setup will perform under heavy load, will this single genserver be a bottleneck?
Some other approaches that I am thinking of are…

Having a pool of worker genservers who can query the ETS table, so under load, some other genserver can pick up messages.
Spawning a genserver for every game, all managed by a DynamicSupervisor. So we have a genserver for each game that can query that game-specific data from ETS. Also, going by this approach should this genserver also create an separate ETS table only for that game, when the game is over the table is deleted

Or any other better approach that you can suggest.

But all these things will add complexity to the code and I only want to consider these if the present setup could be a bottleneck.

Can anyone help me with these approaches and also it will be very helpful if you can provide some code reference or links explaining how to manage genserver pools or dynamically spawning genservers if you are suggesting those solutions.

Thanks!

14 comments

/phoenix #ets #genserver

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2021-06-20 08:11:56 UTC

Most Liked

LostKobrakai

Also be sure to understand the difference between sequencing writes by going through the genserver process for writing to ets vs. having the ets table be public and directly writing to it without involving the genserver starting the table.

Post #3

derek-zhou

All 3 ways are possible with ETS:

read/write exclusively from the a single process
a single process handle all mutations, any processes can read
any processes can read and write

I usually choose the middle ground so my ets table is always consistent and reads are parallel

Post #12

mindok

First things first, can you create a load simulator so you can test performance under different loads? That way you will know what needs fixing when, and whether the fixes are good.

100 games isn’t really a heavy load (depending on frequency of updates per game), but piping all activity through a single process (particularly one that “owns” the ets table) will end in tears before too long - one bad input will crash all games for all users. OTP is all about isolating “conversations” so one failure doesn’t affect thousands or millions of connections.

Your second option (GenServer per game) makes sense. Whether or not your second option is absolutely the best I don’t know, but you should try coding it anyway - you will learn some key lessons along the way and it shouldn’t introduce much complexity overall in return for improved reliability and responsiveness.

This article will give you some pointers: The Erlangelist - To spawn, or not to spawn?

Post #2