Firefox/session restore

What is Session Restore, why this page?

Session Restore is the service that saves the state of Firefox while the user is browsing so as to be able to restore it after a shutdown or a crash.

This service is extremely useful but is also quite expensive. It was designed a long time ago, when users only had a few tabs, when DOM Storage didn't exist and before people started uploading Gigabytes of data. For this reason, it is undergoing major refactorings and redesigns.

This page is about documenting things progressively.

Collecting and saving data

TBD

Data we collect

• Open windows and tabs
• Closed windows and tabs (closed tabs, tabs in closed windows, closed tabs in closed windows)
• Loaded pages before and after (back and forward buttons)
• Form data (for the case of a browser or system crash)
• ... (TBD)

Data we do *not* collect

TBD

Privacy

TBD

Technicality informations

Paths and file

Windows XP: ???
Windows 7: [system-drive]:\Users\[user-name]\AppData\Roaming\Mozilla\Firefox\Profiles\[profile-name[.default]]\
Linux: ???

File: sessionstore.js

Backups:
The browser creates backups.
The files are:
- sessionstore.bak
- sessionstore.bak-[yyyymmddtttttt]

Browser settings

browser.sessionstore.interval:

 Default: 15000ms

browser.sessionstore.max_tabs_undo:

 Default: 10

browser.sessionstore.max_windows_undo:

 Default: 3

browser.sessionhistory.max_entries:

 The maximum number of session history entries to keep in working memory (which includes the 'current' page).
 Default: 50

browser.sessionstore.max_serialize_back:

 The maximum number of 'back button' session history entries to store in sessionstore.js.
 Default: ?? (-1 = no limit)

browser.sessionstore.max_serialize_forward:

 The maximum number of 'forward button' session history entries to store in sessionstore.js.
 Default: ?? (-1 = no limit)

View content

With extension: Add-on: https://addons.mozilla.org/en-US/firefox/addon/about-sessionstore/ Author: https://addons.mozilla.org/en-US/firefox/user/dtryse/

Restoring sessions

TBD

Redesigning sessionstore.js

Our problems

During startup

We need to read + parse everything, even when we restore-on-demand or do not restore at all:

• Slower startup
• Much I/O
• CPU-hungry
• Memory-hungry

During runtime

We need to serialize + write everything, even when only a single tab has changed.

• Slower runtime
• Much I/O
• CPU-hungry
• Memory-hungry

During shutdown

• During shutdown, final serialize + write everything
• Much I/O (annoying)
• Slower shutdown (less annoying)
• CPU-hungry (less annoying)
• Memory-hungry (less annoying)

Improvements

Reduce amount of data stored

Good for: everything.

Limitations: there is only so much data we can cull away.

Difficulty: 2/5.

Compress data

Good for:

• startup I/O;
• runtime I/O;
• shutdown I/O.

Bad for:

• CPU (not much);
• Memory (not much);
• Human readability (a lot).

Difficulty: 1/5.

Journalize files, consolidate them on idle-daily and shutdown

Good for reducing serialize + write:

• runtime I/O;
• runtime CPU use;
• runtime memory use.

Bad for:

• shutdown CPU use (probably not a big deal);
• shutdown memory use;
• runtime I/O, CPU, memory in case of crash;
• addon-compat;
• getBrowserState() & co. use a different path from SessionSaver.run.

The consolidated version would not hurt readability.

Difficulty: 3/5.

Save separately index (one file), tabs (one file per tab)

Good for:

• startup I/O (unless the user);
• startup CPU;
• startup memory;
• startup duration;
• runtime I/O;
• runtime CPU;
• runtime memory;
• runtime duration.

Bad for:

• garbage-collecting files may be difficult;
• addon-compat;
• getBrowserState() & co. use a different path from SessionSaver.run.

Readability would be ok.

Difficulty: 4/5.

Binary format that can be loaded/modified piecewise

Good for:

• startup I/O;
• startup CPU;
• startup memory;
• startup duration;
• runtime I/O;
• runtime CPU;
• runtime memory;
• runtime duration;

Bad for:

• debugging;
• extensibility;
• garbage-collecting will be difficult;
• getBrowserState() & co. use a different path from SessionSaver.run();
• readability.

Difficulty: 5/5.

Sqlite

Good for:

• startup I/O;
• startup CPU;
• startup memory;
• startup duration;
• runtime I/O;
• runtime CPU;
• runtime memory;
• runtime duration.

Bad for:

• backups;
• use of flush() is bad for battery & rest of the system;
• garbage-collection will be moderately difficult;
• getBrowserState() & co. use a different path from SessionSaver.run.

Difficulty: 3/5.

Other DBMS

?

Reducing the amount of data we store

History entries

TBD

DOM Session Storage

TBD

POST data

TBD

Forgetting closed tabs/windows after a time

TBD