Swing and a Miss
When we landed the asynchronous location bar, some people started to see substantially slower results. This was alarming since it was supposed to end up speeding or staying the same for everyone. After some investigation, we realized that the AutoComplete code was doing something very dumb with asynchronous searches. The problem was that the code would not actually handle the user pressing the enter key until the next set of results came in. Not only did this result in slower processing of the user’s selection, it also meant that weird race conditions would come up such as up opening a new tab, pasting a url in, press enter, and then switch tabs resulting in the reloading of the page that was just switched to. In other words, epic fail all around.
Luckily, the fix was trivial. Now we handle the enter keypress immediately when the user hits it, and not later. Problem solved :)
Asynchronous Location Bar has Landed
About two weeks ago the asynchronous location bar work landed in mozilla-central without much issue. It’s also in the Firefox 3.6 alpha we just recently released. This has the potential to impact all of our users, but those on slower hard drives will notice this the most. Your location bar searches may not complete any faster than before, but they certainly won’t be hanging your browser and locking up the UI.
Background
We’ve been getting reports for some time about the location bar hanging the application for some users when they are typing in it. This wasn’t a problem that was reproducible on every machine, and even on machines that saw it, it wasn’t always 100% reproducible. Clearly, this behavior is not desirable, so we set out to fix it.
I had a theory to the cause almost a year ago and filed a bug that I was hoping we could work on and fix for Firefox 3.5. We knew that reading data off a disk can be slow (and certainly would complete in a non-deterministic amount of time). Since SQLite uses blocking read calls (no more code can execute until the data is read from disk), this could certainly be the cause of the slowdown our users were seeing. Some simple profiling showed that this was largely the cause of the hanging. Work began on the project, but it was clear that enough issues were cropping up that we were not going to be able to safely take this change for Firefox 3.5, and resources were diverted elsewhere.
Process and Solution
This section is a bit technical, so feel free to skip it. The short answer is “do not block the main thread while reading from the hard drive.”
In order to not block the main thread while reading from disk we either need to make SQLite use non-blocking read system calls, or call into SQLite off of the main thread. Changing the SQLite code isn’t something we want to do, so that solution was out of the question. Luckily, we had solved a similar problem with writes and fsyncs earlier in the Firefox 3.5 development with the asynchronous Storage API.
The first implementation that we tried essentially did the same thing that the old code did. We would execute a query, but this time asynchronously, and then process the results and see if they match. There were two issues with this approach, however. The first issue was that we were filtering every history and bookmark entry on the main thread for a given search. That could be a lot of work we end up doing, and with the additional overhead of moving data across threads, the common case would see no win. The second issue was that once we selected a result in the location bar, and a search was not yet complete, there would be a hang as the main thread processed a bunch of events that Storage had posted to it containing results.
At this point, we realized we needed to do the filtering on a thread other than the main thread. After some thought, we was figured that the easiest way to do that would be to use a SQL function that we define in the WHERE clause of our autocomplete queries. This way, all the filtering is done on a background thread, and the code that runs on the main thread only deals with results we will actually use. This solution exposed some things in the Storage backend like lock contention and a few other subtle issues, but nothing major came up.
For more details on how the location bar search results are generated, see my explanation here.
If you weren’t having a problem before, chances are you won’t notice any difference at all.
Test Build: Asynchronous Location Bar (Take >2)
I have another test build for folks to try out. This fixes a possible error condition that could happen in certain circumstances. This build has two known issues:
- There is a lot of flickering when new results show up. This is being tracked in bug 393902.
- Your computer will hang for a period of time (it will become responsive again) if you continue to type once no results are found. This is being tracked in bug 503701.
This is built off of a “stable” point of mozilla-central, so it’s like using a 3.6 nightly. All the normal warnings apply about using it. I’m told this greatly increases the speed at which results obtained by many people. If you experience any issues (other than the two listed here), please let me know! The builds can be found here.
Test Build: Asynchronous Location Bar
It’s been a while, but last week I started working on the asynchronous location bar patch again. The add-on from past posts will not be updated as there are binary changes that need to be made. I do, however, have some test builds that you can try out. These are built of off mozilla-central, so it’s like using a 3.6 nightly. That means it may not be stable, and you shouldn’t use this with your default profile (but feel free to copy your places.sqlite file into a new one to really hammer on it). If you are interested in trying it out, the builds can be found here.
Your feedback is appreciated!
Asynchronous Location Bar Searches
For those of you using the asynchronous location bar add-on, today’s nightly of both mozilla-central and mozilla-1.9.1 should show a reduction in memory use. Turns out there was a bit of a leak in mozStorage that would mean you’d get high memory usage that would never go down. My bad.
For those of your programmers out there, I have a word of advice. Always make sure you have a virtual destructor in your base class. If you don’t, you could spend days tracking down a leak that doesn’t seem to make sense. Of course, this probably would have been spotted earlier if we reported leaks in xpcshell unit tests.