Ubuntu boot speed fall in Hardy

Krzysztof Lichota krzysiek at lichota.net
Fri May 23 08:59:48 BST 2008


2008/5/22 Scott James Remnant <scott at canonical.com>:
> On Tue, 2008-05-13 at 14:07 +0200, Krzysztof Lichota wrote:
>> The kernel module writes it. For boot prefetching, userspace script
>> processes the lists as they are merged and sorted for last 3 runs.
>>
> I noticed that you get lists (in /) for the phases, but files
> in /prefetch for applications named PATH-stamp?

Yes, boot prefetch files are in /.prefetch-boot-trace.PHASE (I should
change that), while application files and historical boot files are
kept in /prefetch.

> Could you give a little more detail on what files to expect, and what
> the content/format of those files are?

You should expect:
/.prefetch-boot-trace.PHASE
/prefetch/.prefetch-boot-trace.PHASE.TIMESTAMP (3 files for each phase)
/prefetch/APPNAME-HASH for each application using prefetching

Prefetch file format is simple, the header and then series of trace records.
You can see the structures in file prefetch_types.h in prefetch
userspace tools source.

Header structure:
typedef struct {
        ///Trace file signature - should contain trace_file_magic
        char magic[4];
        ///Major version of trace file format
        u16 version_major;
        ///Minor version of trace file format
        u16 version_minor;
        ///Trace raw data start
        u16 data_start;
} prefetch_trace_header_t;

Trace record:
typedef struct {
        kdev_t device;
        unsigned long inode_no;
        pgoff_t range_start;
        pgoff_t range_length;
} prefetch_trace_record_t;

You can print the contents of the trace using "prefetch-print-trace"
utility included in prefetch userspace tools.

>> Init scripts (similar to readahead scripts) are run and they tell
>> kernel module which files to load and when.
>> So boot prefetching can be easily changed by modifying these scripts,
>> without touching the kernel part.
>>
> I noticed the phases stuff.
>
> Have you considered instead using cgroups to collate them?  Phases are
> divided by time, which becomes problematic with a boot sequence running
> in parallel.
>
> A cgroups subsystem for prefetch would solve this, since cgroups are
> inherited from parent to child.
>
> E.g.
>
>  * rcS is placed into the "boot" cgroup
>   (thus all apps run by it are)
>  * rc2 is placed into the "system" cgroup
>  * gdm is placed into the "gui" cgroup
>
> You can then still generate app prefetch lists for individual apps
> (since apache can be started by hand, _and_ by rc2).  But also we can
> generate combined lists for each cgroup.

When I was writing this, cgroups were not available. But it seems like
a good idea. Can you tell something more how to use them?

CCing prefetch-devel mailing list, I think this list would be better
for further discussions as it is getting too much into details for
ubuntu-devel-discuss.

-- 

	Krzysztof Lichota



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