Rusty Russell's Coding Blog

I will freely admit that kernel work has dropped in my priority list. But that didn’t excuse sloppy work like my ae1b22f6e46 commit which sought to sidestep an issue for lguest without me having to do much work.

There’s a 64 bit atomic exchange instruction on x86 called cmpxchg8b.  This isn’t supported on pre-586 processors, so we have cmpxchg8b_emu, but that implementation isn’t paravirtualized so won’t run under lguest.  That’s OK, we used to never run it except on machines which didn’t support that cmpxchg8b instruction and I’ve never received a report.  Then at some point we started doing so: the easiest mod I could see was to switch emulation off except for kernels specifically compiled for 386 or 486.  But I missed Linus’ commit which had set the archs on which emulation was skipped:

Note, this patch only adds CMPXCHG8B for the obvious Intel CPU’s, not for others. (There was something really messy about cmpxchg8b and clone CPU’s, so if you enable it on other CPUs later, do it carefully.)

Now, I could blame Linus for putting that in a commit message, not in the Kconfig.cpu file where anyone changing it was going to see it.  But you should always double-check when you think you’re “fixing” something, and I didn’t.

(I get annoyed when developers don’t detail exactly what commit introduced a problem: it’s not just for convenient reading, but such research often prevents reintroducing subtle bugs! Like, say, Cyrix 6×86 not booting…)

So, with my recent evangelizing of Bloom Filters, Tridge decided to try to apply them on a problem he was having.  An array of several thousand of unsorted strings, each maybe 100 bytes, which needed checking for duplicates.  In the normal case, we’d expect few or no duplicates.

A Bloom Filter for this is quite simple: Wikipedia tells you how to calculate the optimal number of hashes to use and the optimal number of bits given (say) a 1 in a million chance of a false positive.

I handed Tridge some example code and he put it in alongside a naive qsort implementation.  It’s in his junkcode dir.  The result?  qsort scales better, and is about 100x faster.  The reason?  Sorting usually only has to examine the first few characters, but creating N hashes means (in my implementation using the always-awesome Jenkins lookup3 hash) passing over the whole string N/2 times.  That’s always going to lose: even if I coded a single-pass multihash, it’s still having to look at the whole string.

Sometimes, simplicity and standard routines are not just clearer, but faster.

I haven’t used an iPhone in anger so I can’t compare, but I got this so I could use Google Maps to navigate public transport: Adelaide’s integration is excellent, and as I have no car it’s important for Arabella and me.

The Good

• Typing on anything that size is suboptimal, but clever auto predict and correct are well done.
• Google maps integration is nice: knowing my location makes getting public transport directions really sweet.
• Since I pay 15c/MB 1.5c/MB for data on the phone network, grabbing onto my home WiFi network where possible is good.
• Google Calendar and contacts storage means I fear obsolete/lost phone much less.
• Plugging in as a USB mass-storage makes transferring music and pictures of Arabella from Linux really easy.
• Some neat features, like turning a map search into a contact (and then easily using a contact when looking for directions).

The Bad

• Intermittent bugs, such as no characters showing up for SMS when the keyboard is in landscape mode, I discovered can be solved by power-cycle.
• My calendar notifications are completely silent.  I’ve played with every option, but when a calendar event comes due, there’s nothing but a glowing trackball to indicate it.  This seems to be a bug, but I’m reluctant to factory reset to see if that fixes it.
• Calendar entries default to My Calendar: if you forget to flip that to your google account when first creating the entry, it won’t be shared.  I want everything mirrored to Google, so several times I’ve had to delete and recreate laboriously-entered appointments.
• I wish the screen would flip faster; landscape makes a better kbd, but portrait often better browsing.
• Sometimes slowness makes it think I held down a key (giving a special character) when I didn’t, and the autopredict/correct seems to give up if you held a key down.
• I have access to Internode’s hotspots around the city, but as that needs a username/password logon, it’s not useful automatically.
• Battery life (in this stage of intense use) is 1-2 days.

I got it from Portagadgets.com, who were efficient (A$487 + $36 shipping, done via local bank transfer).  Getting an account and new SIM from Exetel took longer.

Conclusion: it’s definitely usable by non-geeks, and has raised my expectations of future phones.  There are some things (such as writing this post) which are much easier on my laptop.  But for reading Facebook or Wikipedia, finding your way on Google Maps, or having SMS conversations it’s excellent.

Had an old copy of the “Google Analytics For WordPress” lying around (which didn’t seem to put anything in my blog source), but after upgrading it it kept saying “Google Analytics settings reset to default” whenever I tried to change anything.

See this thread which talks about the problem and waves at the solution.  Here’s what you need to do, if like me you’re not a WordPress/MySQL junkie and want simple instructions:

1. Find your wordpress config file.  Mine, on a Debian system, was in /etc/wordpress/config-rusty.ozlabs.org.php
2. It will contain lines like this:

   define('DB_NAME', 'rustyozlabsorg');

   define('DB_USER', 'rustyozlabsorg');

   define('DB_PASSWORD', 'g1812fbsa');

3. You need to open the mysql database, we’re going to do some brain surgery to remove the old cruft:

   $ mysql --user=rustyozlabsorg --password=g1812fbsa rustyozlabsorg

4. This should give you a “mysql>” prompt, where you type the following:

   DELETE FROM wp_options where option_name='GoogleAnalyticsPP';

5. It should say something like “Query OK, 1 row affected (0.00 sec)”. Then type

   quit;

6. You’re done.  Reload your setting page and try again.

Hope that gets into Google and helps someone else who can’t figure out what’s going on!