PartitionerOptimisation

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This section should include a paragraph describing the end-user impact of this change. It is meant to be included in the release notes of the first release in which it is implemented. (Not all of these will actually be included in the release notes, at the release manager's discretion; but writing them is a useful exercise.)

It is mandatory.
Ubiquity's manual partitioner is now substantially faster.
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== User stories ==

== Assumptions ==
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You can have subsections that better describe specific parts of the issue. Exact optimisation strategies often need to be determined during implementation, but we discussed a number of tools and techniques we might use.

"Scanning disks" is more or less equivalent to pressing Enter on each partition in d-i to fetch information about it, and then going back to the next one. This implies that there are two main avenues of attack: make ubiquity's partitioning wrapper code need to do less work in terms of partman operations, and make partman itself faster.

=== Reducing partman operations in ubiquity ===

The way ubiquity scans all partitions incidentally involves asking partman to "redisplay" the partition tree at each step, even though no changes have been made. We should be able to cache or avoid this for a speed-up on the order of 50%.

Ubiquity's partitioner is a trade-off between maintainability and efficiency. We will identify places where we're asking partman for a relatively small amount of information at high cost, and replace those by using our existing Python bindings to `parted_server`.

Using `cdebconf` rather than Perl `debconf` in ubiquity didn't make a significant speed difference the last time Colin tried it, but we should recheck this. Ubiquity already has a switch for this which would at most just need a small amount of bitrot-fixing. Note, though, that there are some intentional semantic differences between `cdebconf` and `debconf`, particularly in the area of the `seen` flag, so we would need to be very careful of semantics here.

`ntfsresize` and other resizing tools are called quite often to determine resize bounds of partitions. These are expensive, so we'll figure out how the number of calls here can be reduced.

There are a number of places where we rescan all partitions, even though we can determine that only a certain number of partitions need to be rescanned (for example, resizing a partition only affects the resized partition and the part of the disk immediately following it). We will assess and reduce these.

The partition bar construction in cairo is quite slow; it apparently repeatedly calls `os-prober`, and the caching attempt seems not to be working.

There is a long pause after the partitioner while a check script runs `du` to find out the size of `/rofs`. We will do this statically when building the live filesystem instead.

=== Speeding up partman ===

Progress on this in the past has been blocked on the absence of a good shell profiling tool; `strace` is too intrusive and tends to disturb timings too much, and in any case does not make it easy to see the big picture. During the UDS session, `bootchart` was pointed out as a promising tool here. We'll apply this to manual partitioning sessions. The main focus should be on `choices` scripts in `/lib/partman` and their descendants, since those are used to generate menus and inefficiencies here will be very noticeable.
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This section should describe a plan of action (the "how") to implement the changes discussed. Could include subsections like:
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Should cover changes required to the UI, or specific UI that is required to implement this

=== Code Changes ===

Code changes should include an overview of what needs to change, and in some cases even the specific details.

=== Migration ===

Include:
 * data migration, if any
 * redirects from old URLs to new ones, if any
 * how users will be pointed to the new way of doing things, if necessary.
Even with speed increases, the pop-up progress window is jarring. We will follow up on Bug:336751 to restyle this.
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It's important that we are able to test new features, and demonstrate them to users. Use this section to describe a short plan that anybody can follow that demonstrates the feature is working. This can then be used during testing, and to show off after release. Please add an entry to http://testcases.qa.ubuntu.com/Coverage/NewFeatures for tracking test coverage. We'll designate a test system for timings, which will probably be a virtual machine on Colin's laptop (though of course anyone can do similar timings from Karmic->Lucid on some other system). The most important attribute is the partition layout, and we already know that the partitioner is slow when there are lots of partitions, so we will time a system containing two disks with eight partitions each.
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This need not be added or completed until the specification is nearing beta.

== Unresolved issues ==

This should highlight any issues that should be addressed in further specifications, and not problems with the specification itself; since any specification with problems cannot be approved.

== BoF agenda and discussion ==

{{{
"Scanning disks"
Equivalent to pressing Enter on each partition and going back, in d-i
This clearly involves redisplaying the partition tree at each step, so may be able to cache that part

bootchart for shell profiling?
focus on 'choices' scripts in /lib/partman/; slowness here will be noticeable on generating menus in partman

cdebconf:
 * last time Colin tried, didn't make a significant speed difference; should recheck this
 * doesn't support 'seen' flag, so be careful about semantics (particularly around language/timezone/keymap)

Use bootchart for all of d-i, or all of the live CD.
Bootchart times out; will need magic runes to keep it running.

Use python bindings to libparted?

We're calling ntfsresize multiple times.

Unrelated, but worth noting:
 * Use a status bar rather than a pop-up progress window.

Partition bar construction in cairo is fairly slow (evand)
 * repeated calls out to `os-prober`, which is slow
 * `os-prober` should use `blkid` to discover filesystem types, rather than attempting lots of mounts

Big pause after partitioner while a check script runs du
 * Find out the size of /rofs when ubiquity starts, and plug that into the sufficient free space check and the total_size variable in scripts/install.py
 * if df is reporting incorrect sizes, it should be fixed and ubiquity should use that for determining sizes.

Goal: 20% of current time for two disks with eight partitions each (*handwave*)
}}}
A sensible goal seems to be to get the time for "Scanning disks" on such a system down to 20% of the time in Karmic.

Summary

ubiquity's partitioner is kind of slow, especially with lots of disks/partitions. This is partly due to slowness in the underlying partitioner that's also visible in d-i (though less annoying there) and partly due to inefficiencies in the ubiquity integration. Let's make it be more pleasant to use.

(Note that this is mainly about the manual partitioner, although some effects would likely ripple out to the automatic partitioner as well.)

Release Note

Ubiquity's manual partitioner is now substantially faster.

Rationale

This has been an ever-increasing source of user complaints, and it would be lovely to spend some time optimising it. We just need to make sure that the time spent doing so isn't open-ended, of course.

Design

Exact optimisation strategies often need to be determined during implementation, but we discussed a number of tools and techniques we might use.

"Scanning disks" is more or less equivalent to pressing Enter on each partition in d-i to fetch information about it, and then going back to the next one. This implies that there are two main avenues of attack: make ubiquity's partitioning wrapper code need to do less work in terms of partman operations, and make partman itself faster.

Reducing partman operations in ubiquity

The way ubiquity scans all partitions incidentally involves asking partman to "redisplay" the partition tree at each step, even though no changes have been made. We should be able to cache or avoid this for a speed-up on the order of 50%.

Ubiquity's partitioner is a trade-off between maintainability and efficiency. We will identify places where we're asking partman for a relatively small amount of information at high cost, and replace those by using our existing Python bindings to parted_server.

Using cdebconf rather than Perl debconf in ubiquity didn't make a significant speed difference the last time Colin tried it, but we should recheck this. Ubiquity already has a switch for this which would at most just need a small amount of bitrot-fixing. Note, though, that there are some intentional semantic differences between cdebconf and debconf, particularly in the area of the seen flag, so we would need to be very careful of semantics here.

ntfsresize and other resizing tools are called quite often to determine resize bounds of partitions. These are expensive, so we'll figure out how the number of calls here can be reduced.

There are a number of places where we rescan all partitions, even though we can determine that only a certain number of partitions need to be rescanned (for example, resizing a partition only affects the resized partition and the part of the disk immediately following it). We will assess and reduce these.

The partition bar construction in cairo is quite slow; it apparently repeatedly calls os-prober, and the caching attempt seems not to be working.

There is a long pause after the partitioner while a check script runs du to find out the size of /rofs. We will do this statically when building the live filesystem instead.

Speeding up partman

Progress on this in the past has been blocked on the absence of a good shell profiling tool; strace is too intrusive and tends to disturb timings too much, and in any case does not make it easy to see the big picture. During the UDS session, bootchart was pointed out as a promising tool here. We'll apply this to manual partitioning sessions. The main focus should be on choices scripts in /lib/partman and their descendants, since those are used to generate menus and inefficiencies here will be very noticeable.

Implementation

UI Changes

Even with speed increases, the pop-up progress window is jarring. We will follow up on 336751 to restyle this.

Test/Demo Plan

We'll designate a test system for timings, which will probably be a virtual machine on Colin's laptop (though of course anyone can do similar timings from Karmic->Lucid on some other system). The most important attribute is the partition layout, and we already know that the partitioner is slow when there are lots of partitions, so we will time a system containing two disks with eight partitions each.

A sensible goal seems to be to get the time for "Scanning disks" on such a system down to 20% of the time in Karmic.


CategorySpec

Ubiquity/PartitionerOptimisation (last edited 2010-02-13 03:53:58 by 82-69-40-219)