Re: [gentoo-portage-dev] Re: search functionality in emerge

[Alec Warner]

On Tue, Dec 2, 2008 at 4:42 AM, Tambet <qtvali@×××××.com> wrote:
> About zipping.. Default settings might not really be good idea - i think
> that "fastest" might be even better. Considering that portage tree contains
> same word again and again (like "applications") it needs pretty small
> dictionary to make it much smaller. Decompressing will not be reading from
> disc, decompressing and writing back to disc as in your case probably - try
> decompression to memory drive and you might get better numbers.

I ran gzip -d -c file.gz > /dev/null, which should not write to disk.

I tried again with gzip -1 and it still takes 29ms to decompress (even
with gzip -1) where a bare read takes 26ms.  (I have a 2.6Ghz X2 which
is probably relevant to gzip decompression speed)

>
> I have personally used compression in one c++ application and with optimum
> settings, it made things much faster - those were files, where i had for
> example 65536 16-byte integers, which could be zeros and mostly were; I
> didnt care about creating better file format, but just compressed the whole
> thing.

I'm not saying compression won't make the index smaller.  I'm saying
making the index smaller does not improve performance.  If you have a
10 meg file and you make it 1 meg, you do not increase performance
because you (on average) are not saving enough time reading the
smaller file; since you pay it in decompressing the smaller file
later.

>
> I suggest you to compress esearch db, then decompress it to memory drive and
> give us those numbers - might be considerably faster.

gzip -d -c esearchdb.py.gz > /dev/null (compressed with gzip -1) takes
on average (6 trials, dropped caches between trials) takes 35.1666ms

cat esearchdb.py > /dev/null (uncompressed) takes on average of 6 trials, 24ms.

The point is you use compression when you need to save space (sending
data over the network, or storing large amounts of data or a lot of
something).  The index isn't going to be big (if it is bigger than 20
or 30 meg I'll be surprised), the index isn't going over the network
and there is only 1 index, not say a million indexes (where
compression might actually be useful for some kind of LRU subset of
indexes to meet disk requirements).

Anyway this is all moot since as you stated so well earlier,
optimization comes last, so stop trying to do it now ;)

-Alec

>
> http://www.python.org/doc/2.5.2/lib/module-gzip.html - Python gzip support.
> Try open of that and normal open on esearch db; also compress with the same
> lib to get right kind of file.
>
> Anyway - maybe this compression should be later added and optional.
>
> Tambet - technique evolves to art, art evolves to magic, magic evolves to
> just doing.
>
>
> 2008/12/2 Alec Warner <antarus@g.o>
>>
>> On Mon, Dec 1, 2008 at 4:20 PM, Tambet <qtvali@×××××.com> wrote:
>> > 2008/12/2 Emma Strubell <emma.strubell@×××××.com>
>> >>
>> >> True, true. Like I said, I don't really use overlays, so excuse my
>> >> igonrance.
>> >
>> > Do you know an order of doing things:
>> >
>> > Rules of Optimization:
>> >
>> > Rule 1: Don't do it.
>> > Rule 2 (for experts only): Don't do it yet.
>> >
>> > What this actually means - functionality comes first. Readability comes
>> > next. Optimization comes last. Unless you are creating a fancy 3D engine
>> > for
>> > kung fu game.
>> >
>> > If you are going to exclude overlays, you are removing functionality -
>> > and,
>> > indeed, absolutely has-to-be-there functionality, because noone would
>> > intuitively expect search function to search only one subset of
>> > packages,
>> > however reasonable this subset would be. So, you can't, just can't, add
>> > this
>> > package into portage base - you could write just another external search
>> > package for portage.
>> >
>> > I looked this code a bit and:
>> > Portage's "__init__.py" contains comment "# search functionality". After
>> > this comment, there is a nice and simple search class.
>> > It also contains method "def action_sync(...)", which contains
>> > synchronization stuff.
>> >
>> > Now, search class will be initialized by setting up 3 databases -
>> > porttree,
>> > bintree and vartree, whatever those are. Those will be in self._dbs
>> > array
>> > and porttree will be in self._portdb.
>> >
>> > It contains some more methods:
>> > _findname(...) will return result of self._portdb.findname(...) with
>> > same
>> > parameters or None if it does not exist.
>> > Other methods will do similar things - map one or another method.
>> > execute will do the real search...
>> > Now - "for package in self.portdb.cp_all()" is important here ...it
>> > currently loops over whole portage tree. All kinds of matching will be
>> > done
>> > inside.
>> > self.portdb obviously points to porttree.py (unless it points to fake
>> > tree).
>> > cp_all will take all porttrees and do simple file search inside. This
>> > method
>> > should contain optional index search.
>> >
>> >               self.porttrees = [self.porttree_root] + \
>> >                       [os.path.realpath(t) for t in
>> > self.mysettings["PORTDIR_OVERLAY"].split()]
>> >
>> > So, self.porttrees contains list of trees - first of them is root,
>> > others
>> > are overlays.
>> >
>> > Now, what you have to do will not be harder just because of having
>> > overlay
>> > search, too.
>> >
>> > You have to create method def cp_index(self), which will return
>> > dictionary
>> > containing package names as keys. For oroot... will be
>> > "self.porttrees[1:]",
>> > not "self.porttrees" - this will only search overlays. d = {} will be
>> > replaced with d = self.cp_index(). If index is not there, old version
>> > will
>> > be used (thus, you have to make internal porttrees variable, which
>> > contains
>> > all or all except first).
>> >
>> > Other methods used by search are xmatch and aux_get - first used several
>> > times and last one used to get description. You have to cache results of
>> > those specific queries and make them use your cache - as you can see,
>> > those
>> > parts of portage are already able to use overlays. Thus, you have to put
>> > your code again in beginning of those functions - create index_xmatch
>> > and
>> > index_aux_get methods, then make those methods use them and return their
>> > results unless those are None (or something other in case none is
>> > already
>> > legal result) - if they return None, old code will be run and do it's
>> > job.
>> > If index is not created, result is None. In index_** methods, just check
>> > if
>> > query is what you can answer and if it is, then answer it.
>> >
>> > Obviously, the simplest way to create your index is to delete index,
>> > then
>> > use those same methods to query for all nessecary information - and
>> > fastest
>> > way would be to add updating index directly into sync, which you could
>> > do
>> > later.
>> >
>> > Please, also, make those commands to turn index on and off (last one
>> > should
>> > also delete it to save disk space). Default should be off until it's
>> > fast,
>> > small and reliable. Also notice that if index is kept on hard drive, it
>> > might be faster if it's compressed (gz, for example) - decompressing
>> > takes
>> > less time and more processing power than reading it fully out.
>>
>> I'm pretty sure your mistaken here, unless your index is stored on a
>> floppy or something really slow.
>>
>> A disk read has 2 primary costs.
>>
>> Seek Time: Time for the head to seek to the sector of disk you want.
>> Spin Time: Time for the platter to spin around such that the sector
>> you want is under the read head.
>>
>> Spin Time is based on rpm, so average 7200 rpm / 60 seconds = 120
>> rotations per second, so worst case (you just passed the sector you
>> need) you need to wait 1/120th of a second (or 8ms).
>>
>> Seek Time is per hard drive, but most drives provide average seek
>> times under 10ms.
>>
>> So it takes on average 18ms to get to your data, then you start
>> reading.  The index will not be that large (my esearchdb is 2 megs,
>> but lets assume 10MB for this compressed index).
>>
>> I took a 10MB meg sqlite database and compressed it with gzip (default
>> settings) down to 5 megs.
>> gzip -d on the database takes 300ms, catting the decompressed data
>> base takes 88ms (average of 5 runs, drop disk caches between runs).
>>
>> I then tried my vdb_metadata.pickle from
>> /var/cache/edb/vdb_metadata.pickle
>>
>> 1.3megs compresses to 390k.
>>
>> 36ms to decompress the 390k file, but 26ms to read the 1.3meg file from
>> disk.
>>
>> Your index would have to be very large or very fragmented on disk
>> (requiring more than one seek) to see a significant gain in file
>> compression (gzip scales linearly).
>>
>> In short, don't compress the index ;p
>>
>> >
>> > Have luck!
>> >
>> >>> -----BEGIN PGP SIGNED MESSAGE-----
>> >>> Hash: SHA1
>> >>>
>> >>> Emma Strubell schrieb:
>> >>> > 2) does anyone really need to search an overlay anyway?
>> >>>
>> >>> Of course. Take large (semi-)official overlays like sunrise. They can
>> >>> easily be seen as a second portage tree.
>> >>> -----BEGIN PGP SIGNATURE-----
>> >>> Version: GnuPG v2.0.9 (GNU/Linux)
>> >>> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
>> >>>
>> >>> iEYEARECAAYFAkk0YpEACgkQ4UOg/zhYFuD3jQCdG/ChDmyOncpgUKeMuqDxD1Tt
>> >>> 0mwAn2FXskdEAyFlmE8shUJy7WlhHr4S
>> >>> =+lCO
>> >>> -----END PGP SIGNATURE-----
>> >>>
>> >> On Mon, Dec 1, 2008 at 5:17 PM, René 'Necoro' Neumann <lists@××××××.eu>
>> >> wrote:
>> >>
>> >
>> >
>
>