[pulseaudio-discuss] Understanding PulseSimple Large Memory Allocations

[Sean Greenslade]

On Sun, Oct 03, 2021 at 07:44:13AM +0000, Ryan McClue wrote:
> I'm on an Ubuntu system and want to write to a PCM buffer and play it.
> I created the most basic C program that only calls pa_simple_new().  I
> launched this program into Intel VTune and performed a Memory
> Consumption analysis. It showed that libpulsecommon.so allocates
> ~200MB and pa_simple_new() allocates ~67MB.  Why is so much memory
> allocated?

Hi, Ryan.

There are a few subtleties in Linux with regards to memory usage. First,
I recommend taking a look at the usage through a tool like "top", and
notice that it reports three separete values for memory usage: VIRT,
RES, and SHR. These stand for Virtual, Resident, and Shared. Each of
these behaves a little differently.

VIRT is how many kilobytes of memory address space is currently used by
the application. This includes all memory allocations, memory mapped
files, libraries, and anything else that gets addressed by memory. But
just because something is using memory address space doesn't necessarily
mean that it's using physical memory.

RES is how many kilobytes of physical memory are in use by
the application. This is more or less the "true memory usage" of the
application.

The SHR number is how much shared memory is in use by the application.
This can be things like libraries or shared buffers (which pulse uses to
transfer audio data between processes).

I ran a test of pacat-simple.c on my machine and got the following
values:
VIRT - 339 MB
RES  - 5.3 MB
SHR  - 4.6 MB

So you can see that despite the fact that ~300 MB of address space is
used, only ~5 MB of actual RAM is consumed, and most of that is shared
memory (likely the libraries and IPC audio buffers).

Also, something a little unintuitive about Linux and the way it
allocates memory is that allocations don't actually get assigned to
physical memory until they are used. If you call malloc(), you will get
address space assigned which will add to your VIRT count but your RES
count will stay the same. Once you insert values into those memory
addresses, then the RES count will increase, but only by the amount you
actually fill.  This causes mallocs to be almost no cost memory-wise,
and there's pretty much no harm in over-allocating memory if it's not
used.

In summary, don't worry about the VIRT memory consumed by a process;
it's not the amount of physical RAM used.

> Also, as pulseaudio is what is adopted by Ubuntu, is it
> true that I have no other choice than to use pulseaudio as it owns all
> the sound devices?

This question would be better asked on an Ubuntu-specific list. That
said, if your audio needs are very simple (e.g. only one application
ever playing audio at a time), then it should be possible to disable
pulseaudio and use ALSA directly. 

--Sean