Readahead reloaded
If you compare a non-readahead bootchart with a readahead bootchart , you can clearly see, that most of the services are bound to disk I/O.
There are 2 existing "readahead/prefetching" helpers in Fedora.
- readahead http://git.fedorahosted.org/git/?p=readahead (yum install readahead)
- preload http://preload.sf.net/ (yum install preload)
Ubuntu may switch to prefetch ( https://wiki.ubuntu.com/DesktopTeam/Specs/Prefetch ) but that requires a non-standard kernel module (I don't think, that it makes it upstream).
Preload is adaptive, but gathers information by scanning /proc every 20s. Comparisons showed, that readahead performs better in the boot phase before login.
Readahead modifications
- readahead_early starts before udev in rc.sysinit, reads files needed until gdm login.
- readahead_later starts when the login window is displayed, reads files needed from login until xxx seconds after login.
- readahead-collector runs every boot in the background and automatically adapts the readahead files to the boot sequence and usage profile of the user in the first xxx seconds.
Testcase
boot, login, start firefox, close firefox
Numbers in seconds taken from the bootcharts.
| [1] | [2] | [3] | |
| Desktop-1 | 61 | 56 | 9% |
| Desktop-2 | 52 | 42 | 24% |
| Laptop | 100 | 91 | 10% |
[1] :: without readahead: from start of rc.sysinit until closing of firefox [2] :: with readahead: from start of rc.sysinit until closing of firefox [3] :: speedup: = [1] / [2] - 1
Desktop-1:: Default install from livecd, normal disk Desktop-2:: customized, fast RAID-1 disk (90MB/s) Laptop:: customized, slow disk
Positive Side effects
- Desktop reacts to mouse clicks immediately after icons are visible.
- Firefox loads faster.
- Slicker desktop experience
Problems
- if readahead-early is started right after starting the collector, the collector hasn't initialized yet and does not start the early phase recording, because early already started without it noticing
- triggering readahead-later _exactly_ on login prompt display is not yet possible
- readahead-collector conflicts with auditd, because auditd resets all audit rules
- readahead-collector receives all audit events for readahead (overhead), but ignores them later
- on a really slow disk system, with services optimized, readahead may be an overhead
Todo
gdm/kdm
- gdm/kdm needs an option to run a command (/sbin/readahead_later_start) exactly when the login window is visible to the user
readahead
- readahead-collector needs another mechanism to gather data (wrote a systemtap script to gather data, to determine, what readahead-collector needs http://harald.fedorapeople.org/readahead-collector.stp ) https://bugzilla.redhat.com/show_bug.cgi?id=454452
- readahead-collector needs to run nicely at the end (readlink() bomb) Done
- readahead-collector needs to generate the sorted file list at the end Done
- readahead-collector should not overwrite the old file list, if killed before xxx seconds (fast reboot/shutdown after bootup) Done
- readahead-collector should not wait on readahead-early to start recording for the early phase Done
- readahead-later needs a trigger mechanism from gdm to start (/sbin/readahead_later_start)
- RAC_EXCLUDE="/proc /sys /dev /var/log /var/run /var/lock /home /tmp" needs to be customized
- remove readahead initscripts Done
- determine the time of the readahead-later phase readahead-collector measures. xxx seconds after login/system start??
- readahead-early should be started by upstart Done
Outcome
- even better numbers :-)