Re: Are dumb segments dumb?

[Francesco Mari <ma...@gmail.com>]

While the testing effort on dumb segments is ongoing, I opened
OAK-4659 and attached a patch to it. This change is based on the dumb
segments, and improves the format by implementing logic record IDs.
This way, records can by addressed by a record number instead of using
their offsets inside the segment.

2016-07-27 17:06 GMT+02:00 Michael Dürig <md...@apache.org>:
>
> Looks good! I think we should give this one a spin. Some minor points we
> should keep an eye on before we commit this though:
>
> - does tooling still work with the changes in the segment format? Some of
> them access the segments directly such that expanding the segment header by
> 2 bytes might break them.
>
> - have a look at the micro benchmarks and compare to before.
>
> - remind us to remember ;-) updating the documentation of the segment format
> at some point
>
> - I would like to have something along the lines of the segment size test
> back. Probably not as a unit test but more as a benchmark for record sizes.
> So instead of it failing the build, it would output some numbers which we
> could then graph very much the same way like for performance benchmarks.
>
> Michael
>
>
>
> On 26.7.16 11:47 , Francesco Mari wrote:
>>
>> With my latest commits on this branch [1] I enabled every previously
>> ignored test, fixing them when needed., The only two exceptions are
>> RecordUsageAnalyserTest and SegmentSizeTest, that were simply deleted.
>> I also added a couple of tests to cover the cases that work slightly
>> differently than before.
>>
>> [1]: https://github.com/francescomari/jackrabbit-oak/tree/dumb
>>
>> 2016-07-25 17:48 GMT+02:00 Francesco Mari <ma...@gmail.com>:
>>>
>>> It might be a variation in the process I tried. This shouldn't affect
>>> much the statistics anyway, given that the population sample is big
>>> enough in both cases.
>>>
>>> 2016-07-25 17:46 GMT+02:00 Michael Dürig <md...@apache.org>:
>>>>
>>>>
>>>> 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 <md...@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
>>>>>>> <ma...@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 <md...@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
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>
>

Re: Are dumb segments dumb?

[Michael Dürig <md...@apache.org>]

Thanks Francesco for putting this together. I have yet to look at the 
patch and will comment on the issue re. technicalities.

I'm sure logical record ids will open up new ways to further improve 
segment store gc. They can be seen as a more sophisticated solution to 
the "equality" problem I described in OAK-3348 [1]: "the compaction map 
is serving three different concerns: deduplication, equality (of node 
states across different GC generations) and tracking GC generations."

Michael

[1] 
https://issues.apache.org/jira/browse/OAK-3348?focusedCommentId=15190912&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15190912

On 10.8.16 12:19 , Francesco Mari wrote:
> While the testing effort on dumb segments is ongoing, I opened
> OAK-4659 and attached a patch to it. This change is based on the dumb
> segments, and improves the format by implementing logic record IDs.
> This way, records can by addressed by a record number instead of using
> their offsets inside the segment.
>
> 2016-07-27 17:06 GMT+02:00 Michael D�rig <md...@apache.org>:
>>
>> Looks good! I think we should give this one a spin. Some minor points we
>> should keep an eye on before we commit this though:
>>
>> - does tooling still work with the changes in the segment format? Some of
>> them access the segments directly such that expanding the segment header by
>> 2 bytes might break them.
>>
>> - have a look at the micro benchmarks and compare to before.
>>
>> - remind us to remember ;-) updating the documentation of the segment format
>> at some point
>>
>> - I would like to have something along the lines of the segment size test
>> back. Probably not as a unit test but more as a benchmark for record sizes.
>> So instead of it failing the build, it would output some numbers which we
>> could then graph very much the same way like for performance benchmarks.
>>
>> Michael
>>
>>
>>
>> On 26.7.16 11:47 , Francesco Mari wrote:
>>>
>>> With my latest commits on this branch [1] I enabled every previously
>>> ignored test, fixing them when needed., The only two exceptions are
>>> RecordUsageAnalyserTest and SegmentSizeTest, that were simply deleted.
>>> I also added a couple of tests to cover the cases that work slightly
>>> differently than before.
>>>
>>> [1]: https://github.com/francescomari/jackrabbit-oak/tree/dumb
>>>
>>> 2016-07-25 17:48 GMT+02:00 Francesco Mari <ma...@gmail.com>:
>>>>
>>>> It might be a variation in the process I tried. This shouldn't affect
>>>> much the statistics anyway, given that the population sample is big
>>>> enough in both cases.
>>>>
>>>> 2016-07-25 17:46 GMT+02:00 Michael D�rig <md...@apache.org>:
>>>>>
>>>>>
>>>>> 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 D�rig <md...@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
>>>>>>>> <ma...@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 D�rig <md...@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
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>
>>