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:
https://treeherder.mozilla.org/perf.html#/graphs?series=mozilla-central,1684862,1,4&series=mozilla-central,1684846,1,4&series=mozilla-central,1685133,1,4&series=mozilla-central,1685127,1,4
Explicit allocations:
https://treeherder.mozilla.org/perf.html#/graphs?series=mozilla-inbound,1706218,1,4&series=mozilla-inbound,1706220,1,4&series=mozilla-inbound,1706216,1,4
JS:
https://treeherder.mozilla.org/perf.html#/graphs?series=mozilla-central,1684866,1,4&series=mozilla-central,1685137,1,4&series=mozilla-central,1685131,1,4
* Andrew McCreight created a tool for tracking JS memory usage, and
figuring
out which scripts and objects are responsible for how much of it
(https://bugzil.la/1463569).
* 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
(https://bugzil.la/1471066)
* 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
bi-modal.
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