>

> regards, tom lane

From pgsql-hackers-owner+M11649@postgresql.org Wed Aug 1 15:22:46 2001

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From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>

To: "'pgsql-hackers@postgresql.org'" <pgsql-hackers@postgresql.org>

Subject: [HACKERS] Using POSIX mutex-es

Date: Wed, 1 Aug 2001 12:04:24 -0700

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Status: OR

1. Just changed

TAS(lock) to pthread_mutex_trylock(lock)

S_LOCK(lock) to pthread_mutex_lock(lock)

S_UNLOCK(lock) to pthread_mutex_unlock(lock)

(and S_INIT_LOCK to share mutex-es between processes).

2. pgbench was initialized with scale 10.

SUN WS 10 (512Mb), Solaris 2.6 (I'm unable to test on E4500 -:()

-B 16384, wal_files 8, wal_buffers 256,

checkpoint_segments 64, checkpoint_timeout 3600

50 clients x 100 transactions

(after initialization DB dir was saved and before each test

copyed back and vacuum-ed).

3. No difference.

Mutex version maybe 0.5-1 % faster (eg: 37.264238 tps vs 37.083339 tps).

So - no gain, but no performance loss "from using pthread library"

(I've also run tests with 1 client), at least on Solaris.

And so - looks like we can use POSIX mutex-es and conditional variables

(not semaphores; man pthread_cond_wait) and should implement light lmgr,

probably with priority locking.

Vadim

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From pgsql-hackers-owner+M11790@postgresql.org Sun Aug 5 14:41:34 2001

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From: Bruce Momjian <pgman@candle.pha.pa.us>

Message-ID: <200108051830.f75IUhM25071@candle.pha.pa.us>

Subject: Re: [HACKERS] Idea for nested transactions / savepoints

In-Reply-To: <8173.997022088@sss.pgh.pa.us> "from Tom Lane at Aug 5, 2001 10:34:48

am"

To: Tom Lane <tgl@sss.pgh.pa.us>

Date: Sun, 5 Aug 2001 14:30:43 -0400 (EDT)

cc: PostgreSQL-development <pgsql-hackers@postgresql.org>

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Status: OR

> Bruce Momjian <pgman@candle.pha.pa.us> writes:

> > My idea is that we not put UNDO information into WAL but keep a List of

> > rel ids / tuple ids in the memory of each backend and do the undo inside

> > the backend.

>

> The complaints about WAL size amount to "we don't have the disk space

> to keep track of this, for long-running transactions". If it doesn't

> fit on disk, how likely is it that it will fit in memory?

Sure, we can put on the disk if that is better. I thought the problem

with WAL undo is that you have to keep UNDO info around for all

transactions that are older than the earliest transaction. So, if I

start a nested transaction, and then sit at a prompt for 8 hours, all

WAL logs are kept for 8 hours.

We can create a WAL file for every backend, and record just the nested

transaction information. In fact, once a nested transaction finishes,

we don't need the info anymore. Certainly we don't need to flush these

to disk.

--

Bruce Momjian | http://candle.pha.pa.us

pgman@candle.pha.pa.us | (610) 853-3000

+ If your life is a hard drive, | 830 Blythe Avenue

+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026

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From pgman Sun Aug 5 21:16:32 2001

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From: Bruce Momjian <pgman>

Message-ID: <200108060116.f761GWH11356@candle.pha.pa.us>

Subject: Re: [HACKERS] Idea for nested transactions / savepoints

In-Reply-To: <200108051938.f75Jchi27522@candle.pha.pa.us> "from Bruce Momjian

at Aug 5, 2001 03:38:43 pm"

To: Bruce Momjian <pgman@candle.pha.pa.us>

Date: Sun, 5 Aug 2001 21:16:32 -0400 (EDT)

cc: Tom Lane <tgl@sss.pgh.pa.us>,

PostgreSQL-development <pgsql-hackers@postgresql.org>

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Status: OR

> > Bruce Momjian <pgman@candle.pha.pa.us> writes:

> > >> The complaints about WAL size amount to "we don't have the disk space

> > >> to keep track of this, for long-running transactions". If it doesn't

> > >> fit on disk, how likely is it that it will fit in memory?

> >

> > > Sure, we can put on the disk if that is better.

> >

> > I think you missed my point. Unless something can be done to make the

> > log info a lot smaller than it is now, keeping it all around until

> > transaction end is just not pleasant. Waving your hands and saying

> > that we'll keep it in a different place doesn't affect the fundamental

> > problem: if the transaction runs a long time, the log is too darn big.

>

> When you said long running, I thought you were concerned about long

> running in duration, not large transaction. Long duration in one-WAL

> setup would cause all transaction logs to be kept. Large transactions

> are another issue.

>

> One solution may be to store just the relid if many tuples are modified

> in the same table. If you stored the command counter for start/end of

> the nested transaction, it would be possible to sequential scan the

> table and undo all the affected tuples. Does that help? Again, I am

> just throwing out ideas here, hoping something will catch.

Actually, we need to keep around nested transaction UNDO information

only until the nested transaction exits to the main transaction:

BEGIN WORK;

BEGIN WORK;

COMMIT;

-- we can throw away the UNDO here

BEGIN WORK;

BEGIN WORK;

...

COMMIT

COMMIT;

-- we can throw away the UNDO here

COMMIT;

We are using the outside transaction for our ACID capabilities, and just

using UNDO for nested transaction capability.

--

Bruce Momjian | http://candle.pha.pa.us

pgman@candle.pha.pa.us | (610) 853-3000

+ If your life is a hard drive, | 830 Blythe Avenue

+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026