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 delays the start of auditd now Done
- systemtap is not an option (needs kernel-debuginfo)
- wrote a systemtap script to gather data, to determine, what readahead-collector needs http://harald.fedorapeople.org/readahead-collector.stp )
- strace cannot trace init and reimplementing strace for all archs in upstart does not seem reasonable
- wrote an auditd patch (would contradict the auditd use case to store anything on disk immediately) not needed anymore
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) Done
RAC_EXCLUDE="/proc /sys /dev /var/log /var/run /var/lock /home /tmp" needs to be customized Done
remove readahead initscripts Done
readahead-early should be started by upstart Done

Outcome

even better numbers :-)

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Features/30SecondStartup/ReadAheadReloaded

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