Re: GSoC 2017 Proposal for "Explicitly support predicate locks in index access methods besides btree"

Hi guys,

My name is Anant Khandelwal currently i am pursuing masters from IIT -
Delhi and previously i am a software engineer.

I am particularly interested in working on the project "Explicitly support
predicate locks in index access methods besides b tree".I have gone through

http://hdl.handle.net/2123/5353
http://drkp.net/papers/ssi-vldb12.pdf

to understand how serializable implemented through the use of Snapshot
isolation in PostgreSQL also gone through the codebase get some idea of
different access methods for gin, gist, and hash indexes.
I want to discuss my idea.Then i move to make the proposal

Sorry for late i am busy with the academics.

From my findings the dependencies which create the anomalies in Snapshot
isolation are:

*wr-dependencies*: if T1 writes a version of an object, and T2
reads that version, then T1 appears to have executed before T2
* ww-dependencies*: if T1 writes a version of some object, and
T2 replaces that version with the next version, then T1 appears
to have executed before T2
*ww-dependencies*: if T1 writes a version of some object, and
T2 replaces that version with the next version, then T1 appears
to have executed before T2

where T1 and T2 are the transaction

but due to the rw dependency caused by any insert into the index indexes
other than the btree acquires relation level lock which causes
serialization failure.So the main task is to implement page-level predicate
locking in the remaining core index AMs

* Gist Index

B+tree index acquires predicate locks only on leaf pages as index tuples
with record pointers are stored on leaf pages. But for Gist index, we have
to consider locking at each index level as index tuples can be stored in
buffers associated with internal pages or in leaf pages.
So, the functions where we need to insert a call for

1. PredicateLockPage()

->gistScanPage()
after acquiring a shared lock on a buffer

2.CheckForSerializableConflictIn()

-> gistdoinsert()
after acquiring an exclusive lock on a target buffer and before inserting a
tuple

3. PredicateLockPageSplit()

->gistplacetopage()

If there is not enough space for insertion, we need to copy predicate lock
from an old page to all new pages generated after a successful split
operation.

PredicateLockPageSplit(Relation relation, BlockNumber oldblkno, BlockNumber
a lot) is used by b+-tree where two pages are involved in a split
operation. For Gist index, we can define a similar function where more
than one page can be generated after split operation.

* Gin Index

Gin index consists of a B-tree index over key values, where each key is an
element of some indexed items and each tuple in a leaf page contains either
a posting list if the list is small enough or a pointer to posting tree.

1. PredicateLockPage()

->startscanentry()
before calling collectMatchBitmap()

->scanPostingTree()
after acquiring a shared lock on a leaf page

2.CheckForSerializableConflictIn()

-> ginentryinsert()

->gininsertitempointers()
in case of insertion in a posting tree

3. PredicateLockPageSplit()

-> dataBeginPlacetoPageLeaf()

after calling dataPlacetoPageLeafSplit()

* Hash Index

1. PredicateLockPage()

->hashgettuple()
->_hash_first()
->_hash_readnext()
->_hash_readprev()

2.CheckForSerializableConflictIn()

-> _hash_doinsert()

3. PredicateLockPageSplit()

This is just an idea also i understand that Page level predicate locking
exists in the btree AM, where it required the addition of 17 function calls
to implement, but remains unimplemented in the gin, gist, spgist, brin, and
hash index AMs. So we nned to insert function calls at other places also.

Also tell me can we evaluate the performance on PostgreSQL on the
following workloads

- SIBENCH microbenchMark
- TPC-c++

Also can i use the testing tool reside at src/test/isolation.

or i should use the testing tool dtester from markus wanner

On Sat, Apr 1, 2017 at 7:33 PM, anant khandelwal <anantbietec(at)gmail(dot)com>
wrote:

> Hi guys,
>
> My name is Anant Khandelwal currently i am pursuing masters from IIT -
> Delhi and previously i am a software engineer.
>
> I am particularly interested in working on the project "Explicitly support
> predicate locks in index access methods besides b tree".I have gone through
>
> http://hdl.handle.net/2123/5353
> http://drkp.net/papers/ssi-vldb12.pdf
>
> to understand how serializable implemented through the use of Snapshot
> isolation in PostgreSQL also gone through the codebase get some idea of
> different access methods for gin, gist, and hash indexes.
> I want to discuss my idea.Then i move to make the proposal
>
> Sorry for late i am busy with the academics.
>
> From my findings the dependencies which create the anomalies in Snapshot
> isolation are:
>
> *wr-dependencies*: if T1 writes a version of an object, and T2
> reads that version, then T1 appears to have executed before T2
> * ww-dependencies*: if T1 writes a version of some object, and
> T2 replaces that version with the next version, then T1 appears
> to have executed before T2
> *ww-dependencies*: if T1 writes a version of some object, and
> T2 replaces that version with the next version, then T1 appears
> to have executed before T2
>
> where T1 and T2 are the transaction
>
> but due to the rw dependency caused by any insert into the index indexes
> other than the btree acquires relation level lock which causes
> serialization failure.So the main task is to implement page-level predicate
> locking in the remaining core index AMs
>
> * Gist Index
>
> B+tree index acquires predicate locks only on leaf pages as index tuples
> with record pointers are stored on leaf pages. But for Gist index, we have
> to consider locking at each index level as index tuples can be stored in
> buffers associated with internal pages or in leaf pages.
> So, the functions where we need to insert a call for
>
> 1. PredicateLockPage()
>
> ->gistScanPage()
> after acquiring a shared lock on a buffer
>
> 2.CheckForSerializableConflictIn()
>
> -> gistdoinsert()
> after acquiring an exclusive lock on a target buffer and before inserting a
> tuple
>
>
> 3. PredicateLockPageSplit()
>
> ->gistplacetopage()
>
> If there is not enough space for insertion, we need to copy predicate lock
> from an old page to all new pages generated after a successful split
> operation.
>
> PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
> BlockNumber
> a lot) is used by b+-tree where two pages are involved in a split
> operation. For Gist index, we can define a similar function where more
> than one page can be generated after split operation.
>
> * Gin Index
>
> Gin index consists of a B-tree index over key values, where each key is an
> element of some indexed items and each tuple in a leaf page contains either
> a posting list if the list is small enough or a pointer to posting tree.
>
> 1. PredicateLockPage()
>
> ->startscanentry()
> before calling collectMatchBitmap()
>
> ->scanPostingTree()
> after acquiring a shared lock on a leaf page
>
> 2.CheckForSerializableConflictIn()
>
> -> ginentryinsert()
>
> ->gininsertitempointers()
> in case of insertion in a posting tree
>
>
> 3. PredicateLockPageSplit()
>
> -> dataBeginPlacetoPageLeaf()
>
> after calling dataPlacetoPageLeafSplit()
>
> * Hash Index
>
> 1. PredicateLockPage()
>
> ->hashgettuple()
> ->_hash_first()
> ->_hash_readnext()
> ->_hash_readprev()
>
> 2.CheckForSerializableConflictIn()
>
> -> _hash_doinsert()
>
> 3. PredicateLockPageSplit()
>
>
> This is just an idea also i understand that Page level predicate locking
> exists in the btree AM, where it required the addition of 17 function calls
> to implement, but remains unimplemented in the gin, gist, spgist, brin, and
> hash index AMs. So we nned to insert function calls at other places also.
>
> Also tell me can we evaluate the performance on PostgreSQL on the
> following workloads
>
>
> - SIBENCH microbenchMark
> - TPC-c++
>
>
>
>
>
>
>

On Sat, Apr 1, 2017 at 9:03 AM, anant khandelwal <anantbietec(at)gmail(dot)com> wrote:

> From my findings the dependencies which create the anomalies in Snapshot
> isolation are:
>
> wr-dependencies: if T1 writes a version of an object, and T2
> reads that version, then T1 appears to have executed before T2
> ww-dependencies: if T1 writes a version of some object, and
> T2 replaces that version with the next version, then T1 appears
> to have executed before T2
> ww-dependencies: if T1 writes a version of some object, and
> T2 replaces that version with the next version, then T1 appears
> to have executed before T2

I think you meant to replace one of those ww-dependencies sections
with rw-dependencies -- which are, after all, the ones that cause
all the trouble :-).

> but due to the rw dependency caused by any insert into the index indexes
> other than the btree acquires relation level lock which causes serialization
> failure.

That's both a bit strong and a bit narrow. A better statement might
be that the relation-level predicate locks can lead to false
positive serialization failures. After all, detection of a single
write to the index doesn't all by itself lead to a serialization
failure, and the problem when it does is that it might be a "false
positive" -- in some cases where the serialization failure occurs
there would not actually be a serialization anomaly without that
failure.

> So the main task is to implement page-level predicate locking in the
> remaining core index AMs
>
> * Gist Index
>
> B+tree index acquires predicate locks only on leaf pages

Well, if memory serves, we sometimes need to lock at the relation
level (e.g., an empty index), but as far as *page* locks go, that
seems correct because a scan doesn't exit early during the descent
to the leaf level.

The point is that predicate locks on indexes are intended to lock
gaps between the index entries found. Any index entry read at the
leaf level is dead (but not yet vacuumed away), points to a tuple
not visible to the scanning snapshot, or points to a tuple which
*is* visible to the snapshot. In the first two cases we don't need
any predicate lock; in the last one the lock on the heap tuple
covers things. What we need the locks on the indexes for is to
cover the "gaps" into which later writes may occur.

So the need is to ensure that during any insert of an index tuple
that points to a heap tuple we might have read had it existed at the
time of the index scan, we run into a predicate lock to allow us to
recognize the rw-dependency.

> But for Gist index, we have
> to consider locking at each index level as index tuples can be stored in
> buffers associated with internal pages or in leaf pages.

Yes, because a gist scan can return with a "not found" indication
before it gets to the leaf level.

> So, the functions where we need to insert a call for
>
> 1. PredicateLockPage()
>
> ->gistScanPage()
> after acquiring a shared lock on a buffer
>
> 2.CheckForSerializableConflictIn()
>
> -> gistdoinsert()
> after acquiring an exclusive lock on a target buffer and before inserting a
> tuple
>
>
> 3. PredicateLockPageSplit()
>
> ->gistplacetopage()
>
> If there is not enough space for insertion, we need to copy predicate lock
> from an old page to all new pages generated after a successful split
> operation.
>
> PredicateLockPageSplit(Relation relation, BlockNumber oldblkno, BlockNumber
> a lot) is used by b+-tree where two pages are involved in a split
> operation. For Gist index, we can define a similar function where more
> than one page can be generated after split operation.

We could always call the existing function more than once, but there
might be enough performance benefit to justify a new function with
an array of page references. I would want to do it without changing
the API first, and if time permits benchmark that versus a new
function. Without measurable benefit, I don't want to complicate
the API.

> * Gin Index
>
> Gin index consists of a B-tree index over key values, where each key is an
> element of some indexed items and each tuple in a leaf page contains either
> a posting list if the list is small enough or a pointer to posting tree.
>
> 1. PredicateLockPage()
>
> ->startscanentry()
> before calling collectMatchBitmap()
>
> ->scanPostingTree()
> after acquiring a shared lock on a leaf page
>
> 2.CheckForSerializableConflictIn()
>
> -> ginentryinsert()
>
> ->gininsertitempointers()
> in case of insertion in a posting tree
>
>
> 3. PredicateLockPageSplit()
>
> -> dataBeginPlacetoPageLeaf()
>
> after calling dataPlacetoPageLeafSplit()
>
> * Hash Index
>
> 1. PredicateLockPage()
>
> ->hashgettuple()
> ->_hash_first()
> ->_hash_readnext()
> ->_hash_readprev()
>
> 2.CheckForSerializableConflictIn()
>
> -> _hash_doinsert()
>
> 3. PredicateLockPageSplit()

I have not looked in detail at this point, but that seems plausible
and indicates admirable research effort in drafting this proposal.

> This is just an idea also i understand that Page level predicate locking
> exists in the btree AM, where it required the addition of 17 function calls
> to implement, but remains unimplemented in the gin, gist, spgist, brin, and
> hash index AMs. So we nned to insert function calls at other places also.

Exactly.

> Also tell me can we evaluate the performance on PostgreSQL on the following
> workloads
>
> SIBENCH microbenchMark
> TPC-c++

Those are good, but pgbench (both read-only and read-write loads) is
popular to include because any pg hacker can duplicate the tests.

/docs/current/static/pgbench.html

--
Kevin Grittner

On Sat, Apr 1, 2017 at 9:03 AM, anant khandelwal <anantbietec(at)gmail(dot)com> wrote:

> My name is Anant Khandelwal currently i am pursuing masters from IIT - Delhi
> and previously i am a software engineer.
>
> I am particularly interested in working on the project "Explicitly support
> predicate locks in index access methods besides b tree".

Anant,

Your post was mostly identical (as in copy/paste level identical) to a
post by Shubham Barai four days earlier.

/message-id/CALxAEPvRcJzz0SJ2KB_ghaTRrdEj08rygUrFtr5NUQxc6uTeuQ@mail.gmail.com

/message-id/CAD=a8SBZWGD4U-TASJP0OwTPiLEffHoz5O5eV0Um6digqBVvfg@mail.gmail.com

Unless you can produce convincing proof to the contrary, your proposal
will be disqualified because of plagiarism.

--
Kevin Grittner

---------- Forwarded message ----------
From: anant khandelwal <anantbietec(at)gmail(dot)com>
Date: Mon, Apr 3, 2017 at 11:13 PM
Subject: Re: [HACKERS] GSoC 2017 Proposal for "Explicitly support predicate
locks in index access methods besides btree"
To: Kevin Grittner <kgrittn(at)gmail(dot)com>

First ,I have read from this

https://github.com/postgres/postgres/blob/master/src/
backend/storage/lmgr/README-SSI ------Other Index AM implementation

then i understand that at https://github.com/postgres/postgres/tree/master/
src/backend/access/gist--------

gistplacetopage is the workhorse function that performs one step of the
insertion. If the tuple fits, it inserts it to the given page, otherwise it
splits the page, and constructs the new downlink tuples for the split
pages. The caller must then call gistplacetopage() on the parent page to
insert the downlink tuples.

then i look at how btree does that it does that by having a function -----

https://github.com/postgres/postgres/blob/master/src/backend/access/nbtree/
nbtinsert.c

PredicateLockPageSplit(rel,BufferGetBlockNumber(buf),
BufferGetBlockNumber(rbuf));

then i find how PredicateLockPage is called from --------https://github.com/
postgres/postgres/blob/master/src/backend/access/nbtree/nbtsearch.c

then i find how CheckPredicateLocking() is locking is implemented -----

https://github.com/postgres/postgres/blob/master/src/backend/access/nbtree/
nbtinsert.c

The only conflict predicate locking cares about for indexes is when
* an index tuple insert conflicts with an existing lock. Since the
* actual location of the insert is hard to predict because of the
* random search used to prevent O(N^2) performance when there are
* many duplicate entries, we can just use the "first valid" page.

then gistScanPage() ----- https://github.com/postgres/postgres/blob/master/
src/backend/access/gist/gistget.c

Scan all items on the GiST index page identified by *pageItem, and insert
* them into the queue (or directly to output areas)
So we need to put the predicateLockPage() there

I think i have provided sufficient proof for the Index AM gist if you want
i can give proof for all other ones also.

Thanks
Anant

On Mon, Apr 3, 2017 at 10:03 PM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:

> On Sat, Apr 1, 2017 at 9:03 AM, anant khandelwal <anantbietec(at)gmail(dot)com>
> wrote:
>
> > My name is Anant Khandelwal currently i am pursuing masters from IIT -
> Delhi
> > and previously i am a software engineer.
> >
> > I am particularly interested in working on the project "Explicitly
> support
> > predicate locks in index access methods besides b tree".
>
> Anant,
>
> Your post was mostly identical (as in copy/paste level identical) to a
> post by Shubham Barai four days earlier.
>
> /message-id/CALxAEPvRcJzz0SJ2KB_gh
> aTRrdEj08rygUrFtr5NUQxc6uTeuQ(at)mail(dot)gmail(dot)com
>
> /message-id/CAD=a8SBZWGD4U-TASJP0O
> wTPiLEffHoz5O5eV0Um6digqBVvfg(at)mail(dot)gmail(dot)com
>
> Unless you can produce convincing proof to the contrary, your proposal
> will be disqualified because of plagiarism.
>
> --
> Kevin Grittner
>