Android Boot-time Readahead

Here is information about Android Boot-Time Readahead

Presentation
This is the presentation I gave at ABS 2011.

[[Media:abs2011_bird_readahead.pdf|abs2011_bird_readahead.pdf]]

Results
Bottom line results were an approximate 5% (1.5 seconds) improvement in boot time, using readahead on only a single portion of the boot sequence. A 9% improvement was expected, given the area of the boot sequence that was optimized. So, it didn't help as much as I'd hoped, but did help a little.

More testing is needed:
 * to determine why adjusting the I/O scheduling priority did not have the expected affect
 * to apply readahead to more part of the boot sequence
 * to test other methods to avoid interfering with foreground processing
 * like, manually halting and continuing readahead during the boot, and performing smaller reads

Programs
These are test programs, but there's nothing in them that prevents them from being used for actual systems. readahead does not intrinsically alter any functionality on the system. It only changes the contents of the page cache. Worst case behavior should be that things boot a bit slower. Best case behavior is that boot time improves a lot. Please measure with bootchart and let me know how it goes.

mincore
Mincore is a program I wrote to report the pages in the Linux page cache for requested files.

Here's the source:

[[Media:mincore.c]]

logsync
Logsync is a program to write a log message to both the kernel log buffer (printk), and the Android log buffer. On most systems, the timestamps are different between these two logging systems. This is useful to determine a unified ordering of events reported by these separate systems.

[[Media:logsync.c]]

treadahead
Tim's readahead - This is a test program to experiment with various readahead options. Besides actually performing readahead operations, it includes facilities for recording iowait before and after a readahead sequence, timing the duration of readahead operations, and changing things like the process scheduling priority, I/O scheduler, and I/O scheduling priority.

[[Media:treadahead-1.3.tgz]]

You can build this in context of the Android build system (using the included Android.mk) or using a regular Linux host compiler (using the included Makefile). Note that this would build for a 32-bit x86 system (which is what the Sony Internet TV is).

The source includes readahead.S, which is assembly for adding a wrapper for the readahead syscall on ARM systems. Bionic on ARM is missing the wrapper for the readahead syscall.