Why so many applications allocate incredibly large amount of virtual memory while not using any of it?

Question

I've been watching some weird phenomena in programming for quite some time, since overcommit is enabled by default on linux systems.

It seems to me that pretty much every high level application (eg. application written in high level programming language like Java, Python or C# including some desktop applications written in C++ that use large libraries such as Qt) use insane amount of virtual operating memory. For example, it's normal for web browser to allocate 20GB of ram while using only 300MB of it. Or for a dektop environment, mysql server, pretty much every java or mono application and so on, to allocate tens of gigabytes of RAM.

Why is that happening? What is the point? Is there any benefit in this?

I noticed that when I disable overcommit in linux, in case of a desktop system that actually runs a lot of these applications, the system becomes unusable as it doesn't even boot up properly.

Answer 1

Languages that run their code inside virtual machines (like Java (*), C# or Python) usually assign large amounts of (virtual) memory right at startup. Part of this is necessary for the virtual machine itself, part is pre-allocated to parcel out to the application inside the VM.

With languages executing under direct OS control (like C or C++), this is not necessary. You can write applications that dynamically use just the amount of memory they actually require. However, some applications / frameworks are still designed in such a way that they request a large chunk memory from the operating system once, and then manage the memory themselves, in hopes of being more efficient about it than the OS.

There are problems with this:

• It is not necessarily faster. Most operating systems are already quite smart about how they manage their memory. Rule #1 of optimization, measure, optimize, measure.

• Not all operating systems do have virtual memory. There are some quite capable ones out there that cannot run applications that are so "careless" in assuming that you can allocate lots & lots of "not real" memory without problems.

• You already found out that if you turn your OS from "generous" to "strict", these memory hogs fall flat on their noses. ;-)

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(*) Java, for example, cannot expand its VM once it is started. You have to give the maximum size of the VM as a parameter (-Xmxn). Thinking "better safe than sorry" leads to severe overallocations by certain people / applications.

Answer 2

These applications usually have their own method of memory management, which is optimized for their own usage and is more efficient than the default memory management provided by the system. So they allocate huge memory block, to skip or minimize the effect of the memory management provided by system or libc.