On Thu, Jul 12, 2018 at 12:57:35PM +0200, Emilio Cobos Álvarez wrote:
Thanks for doing this!

Just curious, is there a bug on file to measure excess capacity on nsTArrays and hash tables?

I don't think so, but it's a good idea.

I've actually been thinking on filing a bug to do something similar, to measure cumulative effects of excess padding in certain types since I began looking into bug 1460674, and Sylvestre mentioned that clang-analyzer can generate reports on excess padding.

It would probably be a good idea to try to roll this into the same project.

One nice change coming up on this front is that bug 1402910 will probably allow us to increase the load factors of most of our hashtables without losing performance. Having up-to-date numbers for these things would probably help decide how to prioritize those sorts of bugs.

On 07/10/2018 08:19 PM, Kris Maglione wrote:
Welcome to the first edition of the Fission MemShrink newsletter.[1]

In this edition, I'll sum up what the project is, and why it matters to you. In subsequent editions, I'll give updates on progress that we've made, and areas that we'll need to focus on next.[2]

The Fission MemShrink project is one of the most easily overlooked aspects of Project Fission (also known as Site Isolation), but is absolutely critical to its success. And will require a company- and community-wide effort effort to meet its goals.

The problem is thus: In order for site isolation to work, we need to be able to run *at least* 100 content processes in an average Firefox session. Each of those processes has its own base memory overhead—memory we use just for creating the process, regardless of what's running in it. In the post-Fission world, that overhead needs to be less than 10MB per process in order to keep the extra overhead from Fission below 1GB. Right now, on our best-cast platform, Windows 10, is somewhere between 17 and 21MB. Linux and OS-X hover between 25 and 35MB. In other words, between 2 and 3.5GB for an ordinary session.

That means that, in the best case, we need to reduce the memory we use in content processes by *at least* 7MB. The problem, of course, is that there are only so many places we can cut memory without losing functionality, and even fewer places where we can make big wins. But, there are lots of places we can make small and medium-sized wins.

So, to put the task into perspective, of all of the places we can cut a certain amount of overhead, here are the number of each that we need to fix in order to reach 1MB:

250KB:   4
100KB:  10
75KB:   13
50KB:   20
20KB:   50
10KB:  100
5KB:   200

Now remember: we need to do *all* of these in order to reach our goal. It's not a matter of one 250KB improvement or 50 5KB improvements. It's 4 250KB *and* 200 5KB improvements. There just aren't enough places we can cut 250KB. If we fall short in any of those areas, Project Fission will fail, and Firefox will be the only major browser without site isolation.

But it won't fail, because all of you are awesome, and this is a totally achievable goal if we all throw our effort behind it.

Essentially what this means, though, is that if we identify an area of overhead that's 50KB[3] or larger that can be eliminated, it *has* to be eliminated. There just aren't that many large chunks to remove. They all need to go. And if an area of code has a dozen 5KB chunks that can be eliminated, maybe they don't all have to go, but at least half of them do. The more the better.

To help us triage these issues, we have a tracking bug (https://bugzil.la/memshrink-content), and a per-bug whiteboard tag ([overhead:...]) which gives an estimate of how much per-process overhead we believe fixing that bug would eliminate. Please feel free to add blockers to the tracking bug if you think they're relevant, and to add or update [overhead] tags if you have reasonable estimates.

With all of that said, here's a brief update of the progress we've made so far:

In the past month, unique memory per process[4] has dropped 3-4MB[5], and JS memory usage in particular has dropped 1.1-1.9MB.

Particular credit goes to:

* Eric Rahm added an AWSY test suite to track base content process memory
  (https://bugzil.la/1442361). Results:

   Resident unique: 

   Explicit allocations: 


* Andrew McCreight created a tool for tracking JS memory usage, and figuring
  out which scripts and objects are responsible for how much of it

* Andrew and Nika Layzell also completely rewrote the way we handle XPIDL type   info so that it's statically compiled into the executable and shared between
  all processes (https://bugzil.la/1438688, https://bugzil.la/1444745).

* Felipe Gomes split a bunch of code out of frame scripts so that it could be   lazily loaded only when needed (https://bugzil.la/1467278, ...) and added a   whitelist of JSMs that are allowed to be loaded at content process startup

* I did a bit of this too, and also prevented us from loading some other JSMs   before we need them (https://bugzil.la/1470333, https://bugzil.la/1469719,

* Nick Nethercote made dynamic nsAtoms allocate their string storage inline
  rather than use a refcounted StringBuffer (https://bugzil.la/1447951)

* Emilio Álvarez reduced the amount of memory the Gecko Profiler uses in
  content processes.

* Nathan Froyd fixed our static nsAtom code so it didn't generate static
  initializers (https://bugzil.la/1455178) and reduced the stack size of our
  image decoder threads (https://bugzil.la/1443932).

* Doug Thayer reduced the number of hang monitor threads we start in each
  process (https://bugzil.la/1448040)

* Boris Zbarsky removed a bunch of useless QueryInterface implementations
  (https://bugzil.la/1452862), made our static isInstance methods use less
  memory (https://bugzil.la/1452786), and generally deleted a bunch of
  useless, legacy nsI* interfaces that required us to add extra vtable
  pointers to a lot of DOM object instances.

And your humble author contributed the following:

* Changed our localization string bundles to use shared memory for bundles
  which are loaded into content processes (https://bugzil.la/1470365).
  This bug also adds some helpers which should make it easer to use shared
  memory for more things in the future.

* Made some changes to the script preloader to avoid keeping an unnecessary
  encoded copy of scripts in the content process (https://bugzil.la/1470793),   to drop cached single-use scripts (https://bugzil.la/1471091), and to improve   the set of scripts we load in content processes (https://bugzil.la/1471089).

* Made some smaller optimizations to avoid making copies of strings in
  preference callbacks (https://bugzil.la/1472523), and to remove the XPC
  compilation scope (https://bugzil.la/1442737)

Apologies to anyone I missed.

[1]: Please feel free to read the '.' as a '!' if you're so inclined. I
    generally shy away from exclamation marks.
[2]: If this seems like a massive rip-off of Ehsan's Quantum Flow newsletter
    format, that's because it is. Thanks, Ehsan :)
[3]: 50KB per process, which is to say 5MB across 100 content processes.
[4]: The total memory mapped by each content process which is not shared by
    other processes. Approximately equal to USS.
[5]: It's hard to be precise, since the numbers can be noisy, and are often
dev-platform mailing list
dev-platform mailing list

Kris Maglione
Senior Firefox Add-ons Engineer
Mozilla Corporation

NSS is what you would get if HP Lovecraft wrote crypto code.

dev-platform mailing list

Reply via email to