Shared libraries [LWN.net]

Shared libraries

Posted Nov 28, 2025 10:25 UTC (Fri) by bluca (subscriber, #118303)
In reply to: Shared libraries by Cyberax
Parent article: APT Rust requirement raises questions

You _really_ need to go out of your way to get glibc and other core system libraries paged out, on a system that is actually getting used. Of course if it's all sitting idle doing nothing then anything hardly matters.

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Shared libraries

Posted Nov 28, 2025 10:43 UTC (Fri) by farnz (subscriber, #17727) [Link] (5 responses)

You really, really don't if you're RAM constrained (welcome to embedded!).

The kernel is not operating at the level of entire files; it's operating at the page level. Pages that contain relocations are only shared up until the relocation is overwritten by the dynamic linker; pages where the only data used at runtime is relocation metadata are only used up until all the relocations have been handled, at which point they become unused pages.

You can thus have 90% of glibc in RAM, but the critical parts for process startup are not, and you have to do small I/Os to get the missing pages.

Shared libraries

Posted Nov 28, 2025 11:01 UTC (Fri) by bluca (subscriber, #118303) [Link] (4 responses)

That can only happen if you somehow have a system that is both incredibly busy, but also incredibly idle, so that resources are fully saturated and things get aggressively evicted from the page cache, but somehow the system is completely static otherwise. IE, you need to go out of your way to artificially create such a situation. On a normal, busy Linux systems with socket activated services, timer activated services, dbus activated services, etc etc, there are pretty much always processes being started and stopped. If new processes are started so _rarely_ that you don't even have glibc in the page cache, then obviously starting processes is not a bottleneck and it doesn't matter anyway one way or the other.

I'm not really sure why you are trying to conjure up such a contrived and unlikely example just to prove a point? It's not really working

Shared libraries

Posted Nov 28, 2025 11:08 UTC (Fri) by farnz (subscriber, #17727) [Link] (3 responses)

No - it happens when the system is continuously busy, but new process starting is on the "once every 15 minutes" scale, not "every few seconds".

And I do have glibc in the page cache - I just don't have the stuff that's only loaded on process starting in cache.

But I get it, my experience and yours don't match, so you're going to tell me I'm wrong, rather than address a real situation.

Shared libraries

Posted Nov 28, 2025 13:40 UTC (Fri) by LtWorf (subscriber, #124958) [Link] (2 responses)

I think that even if it does happen on some specially built devices with some very specific workloads, it would be counterproductive to optimise for that and make the common case slower for everyone else in the world instead.

Shared libraries

Posted Nov 29, 2025 20:53 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link] (1 responses)

But we still need to support obsolete hardware!

FWIW, static binaries help with responsiveness for most users. I encourage everyone here to try the fully static distro that I mentioned ( . It really feels more snappy.

Shared libraries

Posted Nov 29, 2025 22:08 UTC (Sat) by LtWorf (subscriber, #124958) [Link]

It's not "obsolete", it's just custom built. And it does work just fine with shared libraries for most workloads except whatever weird thing farnz is doing with it.

Feeling more snappy at 2 minutes after boot doesn't necessarily mean it's more snappy 5 days later.

Shared libraries

Posted Nov 30, 2025 1:28 UTC (Sun) by Cyberax (✭ supporter ✭, #52523) [Link] (3 responses)

> You _really_ need to go out of your way to get glibc and other core system libraries paged out

Nope. glibc is loaded at a random location, and when it needs to be linked into an executable, the OS needs to do relocations to resolve the addresses. This information can be easily paged out, especially for rarely used binaries.

glibc by itself is not too large, but when you add other libraries like libstdc++, libz, libsystemd, and others it starts adding up. This is compounded by libraries that use NSS plugins, because dlopen() requires new relocations each time (AFAIR?).

Shared libraries

Posted Dec 1, 2025 11:10 UTC (Mon) by paulj (subscriber, #341) [Link] (1 responses)

Does this imply the static binaries you advocate for are a lot less secure, cause - other than perhaps the initial base address of where it is loaded - all the other addresses are a known entity? Does it not significantly reduce security, by largely negating ALSR? (So far as ALSR provides security - I'm aware there is the odd bit of dissent on the merits).

Shared libraries

Posted Dec 1, 2025 18:22 UTC (Mon) by Cyberax (✭ supporter ✭, #52523) [Link]

Not with the PIC (Position-Independent Code). The kernel itself does relocations when the binary is loaded.

Shared libraries

Posted Dec 1, 2025 13:16 UTC (Mon) by malmedal (subscriber, #56172) [Link]

> OS needs to do relocations

It would be good to properly measure how big this effect is. I did a very quick test running chrome under perf and immediately killing it when it finished starting up. On my machine with a hot cache ld-linux used 7% of cpu-time. This does not prove there is a problem, but it is indicative that it is worth investigating properly.