Interesting numbers. Most of them look as I would have expected. I.e.
the distributions in the dumb case are more regular (smaller std. dev,
mean and median closer to each other), bigger segment sizes, etc.
What I don't understand is the total number of records. These numbers
differ greatly between current and dumb. Is this a test artefact (i.e.
test not reproducible) or are we missing out on something.
Michael
On 25.7.16 4:01 , Francesco Mari wrote:
I put together some statistics [1] for the process I described above.
The "dumb" variant requires more segments to store the same amount of
data, because of the increased size of serialised record IDs. As you
can see the amount of records per segment is definitely lower in the
dumb variant.
On the other hand, ignoring the growth of segment ID reference table
seems to be a good choice. As shown from the segment size average,
dumb segments are usually fuller that their counterpart. Moreover, a
lower standard deviation shows that it's more common to have full dumb
segments.
In addition, my analysis seems to have found a bug too. There are a
lot of segments with no segment ID references and only one record,
which is very likely to be the segment info. The flush thread writes
every 5 seconds the current segment buffer, provided that the buffer
is not empty. It turns out that a segment buffer is never empty, since
it always contains at least one record. As such, we are currently
leaking almost empty segments every 5 seconds, that waste additional
space on disk because of the padding required by the TAR format.
[1]:
https://docs.google.com/spreadsheets/d/1gXhmPsm4rDyHnle4TUh-mtB2HRtRyADXALARRFDh7z4/edit?usp=sharing
2016-07-25 10:05 GMT+02:00 Michael Dürig <mdue...@apache.org>:
Hi Jukka,
Thanks for sharing your perspective and the historical background.
I agree that repository size shouldn't be a primary concern. However, we
have seen many repositories (especially with an external data store) where
the content is extremely fine granular. Much more than in an initial content
installation of CQ (which I believe was one of the initial setup for
collecting statistics). So we should at least understand the impact of the
patch in various scenarios.
My main concern is the cache footprint of node records. Those are made up of
a list of record ids and would thus grow by a factor of 6 with the current
patch.
Locality is not so much of concern here. I would expect it to actually
improve as the patch gets rid of the 255 references limit of segments. A
limit which in practical deployments leads to degeneration of segment sizes
(I regularly see median sizes below 5k). See OAK-2896 for some background on
this.
Furthermore we already did a big step forward in improving locality in
concurrent write scenarios when we introduced the SegmentBufferWriterPool.
In essence: thread affinity for segments.
We should probably be more carefully looking at the micro benchmarks. I
guess we neglected this part a bit in the past. Unfortunately CI
infrastructure isn't making this easy for us... OTOH those benchmarks only
tell you so much. Many of the problems we recently faced only surfaced in
the large: huge repos, high concurrent load, many days of traffic.
Michael
On 23.7.16 12:34 , Jukka Zitting wrote:
Hi,
Cool! I'm pretty sure there are various ways in which the format could be
improved, as the original design was based mostly on intuition, guided
somewhat by collected stats <http://markmail.org/message/kxe3iy2hnodxsghe>
and
the micro-benchmarks <https://issues.apache.org/jira/browse/OAK-119> used
to optimize common operations.
Note though that the total size of the repository was not and probably
shouldn't be a primary metric, since the size of a typical repository is
governed mostly by binaries and string properties (though it's a good idea
to make sure you avoid things like duplicates of large binaries). Instead
the rationale for squeezing things like record ids to as few bytes as
possible is captured in the principles listed in the original design doc
<http://jackrabbit.apache.org/oak/docs/nodestore/segmentmk.html>:
- Compactness. The formatting of records is optimized for size to
reduce
IO costs and to fit as much content in caches as possible. A node
stored in
SegmentNodeStore typically consumes only a fraction of the size it
would as
a bundle in Jackrabbit Classic.
- Locality. Segments are written so that related records, like a node
and its immediate children, usually end up stored in the same segment.
This
makes tree traversals very fast and avoids most cache misses for
typical
clients that access more than one related node per session.
Thus I would recommend keeping an eye also on benchmark results in
addition
to raw repository size when evaluating possible improvements. Also, the
number and size of data segments are good size metrics to look at in
addition to total disk usage.
BR,
Jukka Zitting
On Fri, Jul 22, 2016 at 5:55 AM Francesco Mari <mari.france...@gmail.com>
wrote:
The impact on repository size needs to be assessed with more specific
tests. In particular, I found RecordUsageAnalyserTest and
SegmentSizeTest unsuitable to this task. It's not a coincidence that
these tests are usually the first to be disabled or blindly updated
every time a small fix changes the size of the records.
Regarding GC, the segment graph could be computed during the mark
phase. Of course, it's handy to have this information pre-computed for
you, but since the record graph is traversed anyway we could think
about dynamically reconstructing the segment graph when needed.
There are still so many questions to answer, but I think that this
simplification exercise can be worth the effort.
2016-07-22 11:34 GMT+02:00 Michael Dürig <mdue...@apache.org>:
Hi,
Neat! I would have expected a greater impact on the size of the segment
store. But as you say it probably all depends on the binary/content
ratio. I
think we should look at the #references / repository size ratio for
repositories of different structures and see how such a number differs
with
and without the patch.
I like the patch as it fixes OAK-2896 while at the same time reducing
complexity a lot.
OTOH we need to figure out how to regain the lost functionality (e.g.
gc)
and asses its impact on repository size.
Michael
On 22.7.16 11:32 , Francesco Mari wrote:
Hi,
Yesterday I took some time for a little experiment: how many
optimisations can be removed from the current segment format while
maintaining the same functionality?
I made some work in a branch on GitHub [1]. The code on that branch is
similar to the current trunk except for the following changes:
1. Record IDs are always serialised in their entirety. As such, a
serialised record ID occupies 18 bytes instead of 3.
2. Because of the previous change, the table of referenced segment IDs
is not needed anymore, so I removed it from the segment header. It
turns out that this table is indeed needed for the mark phase of
compaction, so this feature is broken in that branch.
Anyway, since the code is in a runnable state, I generated some
content using the current trunk and the dumber version of
oak-segment-tar. This is the repository created by the dumb
oak-segment-tar:
524744 data00000a.tar
524584 data00001a.tar
524688 data00002a.tar
460896 data00003a.tar
8 journal.log
0 repo.lock
This is the one created by the current trunk:
524864 data00000a.tar
524656 data00001a.tar
524792 data00002a.tar
297288 data00003a.tar
8 journal.log
0 repo.lock
The process that generates the content doesn't change between the two
executions, and the generated content is coming from a real world
scenario. For those familiar with it, the content is generated by an
installation of Adobe Experience Manager.
It looks like that the size of the repository is not changing so much.
Probably the de-optimisation in the small is dwarfed by the binary
content in the large. Another effect of my change is that there is no
limit on the number of referenced segment IDs per segment, and this
might allow segments to pack more records than before.
Questions apart, the clear advantage of this change is a great
simplification of the code. I guess I can remove some lines more, but
what I peeled off is already a considerable amount. Look at the code!
Francesco
[1]: https://github.com/francescomari/jackrabbit-oak/tree/dumb
