Thread: Parallel Vacuum

From:
Dimitri
Date:

Folks,

is there any constrains/problems/etc. to run several vacuum processes in
parallel while each one is 'vaccuming' one different table?

Example:

  vacuum -d db1 -t table1 &
  vacuum -d db1 -t table2 &
  vacuum -d db1 -t table3 &
  wait

(sorry if it was already asked, but I did not find an explicit
answer in archives)

Thanks for any inputs!

Rgds,
-Dimitri

From:
Alvaro Herrera
Date:

Dimitri escribió:
> Folks,
>
> is there any constrains/problems/etc. to run several vacuum processes in
> parallel while each one is 'vaccuming' one different table?

No, no problem.  Keep in mind that if one of them takes a very long
time, the others will not be able to remove dead tuples that were
killed while the long vacuum was running -- unless you are in 8.2.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

From:
Dimitri
Date:

On Thursday 22 March 2007 14:52, Alvaro Herrera wrote:
> Dimitri escribió:
> > Folks,
> >
> > is there any constrains/problems/etc. to run several vacuum processes in
> > parallel while each one is 'vaccuming' one different table?
>
> No, no problem.  Keep in mind that if one of them takes a very long
> time, the others will not be able to remove dead tuples that were
> killed while the long vacuum was running -- unless you are in 8.2.

Yes, I'm using the last 8.2.3 version. So, will they *really* processing in
parallel, or will block each other step by step?

Rgds,
-Dimitri

From:
Alvaro Herrera
Date:

Dimitri escribió:
> On Thursday 22 March 2007 14:52, Alvaro Herrera wrote:
> > Dimitri escribió:
> > > Folks,
> > >
> > > is there any constrains/problems/etc. to run several vacuum processes in
> > > parallel while each one is 'vaccuming' one different table?
> >
> > No, no problem.  Keep in mind that if one of them takes a very long
> > time, the others will not be able to remove dead tuples that were
> > killed while the long vacuum was running -- unless you are in 8.2.
>
> Yes, I'm using the last 8.2.3 version. So, will they *really* processing in
> parallel, or will block each other step by step?

They won't block.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

From:
Dimitri
Date:

On Thursday 22 March 2007 16:12, Alvaro Herrera wrote:
> Dimitri escribió:
> > On Thursday 22 March 2007 14:52, Alvaro Herrera wrote:
> > > Dimitri escribió:
> > > > Folks,
> > > >
> > > > is there any constrains/problems/etc. to run several vacuum processes
> > > > in parallel while each one is 'vaccuming' one different table?
> > >
> > > No, no problem.  Keep in mind that if one of them takes a very long
> > > time, the others will not be able to remove dead tuples that were
> > > killed while the long vacuum was running -- unless you are in 8.2.
> >
> > Yes, I'm using the last 8.2.3 version. So, will they *really* processing
> > in parallel, or will block each other step by step?
>
> They won't block.

Wow! Excellent! :)
So, in this case why not to add 'parallel' option integrated directly into
the 'vacuumdb' command?

In my case I have several CPU on the server and quite powerful storage box
which is not really busy with a single vacuum. So, my idea is quite simple -
speed-up vacuum with parallel execution (just an algorithm):

--------------------------------------------------------------------------
PLL=parallel_degree
select tab_size, tabname, dbname from ... order by tab_size desc;
  vacuumdb -d $dbname -t $tabname  2>&1 > /tmp/vac.$dbname.$tabname.log &
  while (pgrep vacuumdb | wc -l ) >= $PLL
   sleep 1
  end
end
wait
--------------------------------------------------------------------------

biggest tables are vacuumed first, etc.

But of course it will be much more cool to have something like:

   vacuumdb -a -P parallel_degree

What do you think? ;)

Rgds,
-Dimitri

From:
Alvaro Herrera
Date:

Dimitri escribió:

> But of course it will be much more cool to have something like:
>
>    vacuumdb -a -P parallel_degree
>
> What do you think? ;)

I think our time is better spent enhancing autovacuum ... but if you
feel like improving vacuumdb, be my guest.  This discussion belongs into
pgsql-hackers though, and any patches you may feel like submitting for
review should go to pgsql-patches.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

From:
Michael Stone
Date:

On Thu, Mar 22, 2007 at 04:55:02PM +0100, Dimitri wrote:
>In my case I have several CPU on the server and quite powerful storage box
>which is not really busy with a single vacuum. So, my idea is quite simple -
>speed-up vacuum with parallel execution (just an algorithm):

Vacuum is I/O intensive, not CPU intensive. Running more of them will
probably make things slower rather than faster, unless each thing you're
vacuuming has its own (separate) disks. The fact that your CPU isn't
pegged while vacuuming suggests that your disk is already your
bottleneck--and doing multiple sequential scans on the same disk will
definitely be slower than doing one.

Mike Stone

From:
Dimitri
Date:

Mike,

you're right until you're using a single disk :)
Now, imagine you have more disks - more I/O operations you may perform, and
you'll need also a CPU time to process them :)  until you fully use one CPU
per 'vacuumdb' - and then you stop...

As well, even in case when CPU is not highly used by vacuumdb - single process
is still not able to get a max performance of the storage array, just because
you need several concurrent I/O running in the system to reach max
throughput. And even filesystem might help you here - it's not all... More
concurrent writers you have - higher performance you reach (until real
limit)...

In my case I have a small storage array capable to give you more than
500MB/sec and say 5000 op/s. All my data are striped throw all array disks.
Single 'vacuumdb' process here become more CPU-bound rather I/O as it cannot
fully load storage array... So, more vacuum processes I start in parallel -
faster I'll finish database vacuuming.

Best regards!
-Dimitri


On Thursday 22 March 2007 18:10, Michael Stone wrote:
> On Thu, Mar 22, 2007 at 04:55:02PM +0100, Dimitri wrote:
> >In my case I have several CPU on the server and quite powerful storage box
> >which is not really busy with a single vacuum. So, my idea is quite simple
> > - speed-up vacuum with parallel execution (just an algorithm):
>
> Vacuum is I/O intensive, not CPU intensive. Running more of them will
> probably make things slower rather than faster, unless each thing you're
> vacuuming has its own (separate) disks. The fact that your CPU isn't
> pegged while vacuuming suggests that your disk is already your
> bottleneck--and doing multiple sequential scans on the same disk will
> definitely be slower than doing one.
>
> Mike Stone
>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq

From:
Michael Stone
Date:

On Thu, Mar 22, 2007 at 07:24:38PM +0100, Dimitri wrote:
>you're right until you're using a single disk :)
>Now, imagine you have more disks

I do have more disks. I maximize the I/O performance by dedicating
different sets of disks to different tables. YMMV. I do suggest watching
your I/O rates and wallclock time if you try this to see if your
aggregate is actually substantially faster than the single case. (I
assume that you haven't yet gotten far enough to actually do performance
testing.) You may also want to look into tuning your sequential I/O
performance.

Mike Stone

From:
Dimitri
Date:

On Thursday 22 March 2007 19:46, Michael Stone wrote:
> On Thu, Mar 22, 2007 at 07:24:38PM +0100, Dimitri wrote:
> >you're right until you're using a single disk :)
> >Now, imagine you have more disks
>
> I do have more disks. I maximize the I/O performance by dedicating
> different sets of disks to different tables. YMMV. I do suggest watching
> your I/O rates and wallclock time if you try this to see if your
> aggregate is actually substantially faster than the single case. (I
> assume that you haven't yet gotten far enough to actually do performance
> testing.) You may also want to look into tuning your sequential I/O
> performance.
>
> Mike Stone

Mike, specially for you :)

Parallel Vacuum Test
======================

- Database 'db_OBJ'
   PgSQL 8.2.3
   tables: object1, object2, ... object8 (all the same)
   volume: 10.000.000 rows in each table, 22GB in total

- Script Mono Vacuum
  $ cat vac_mono.sh
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object1
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object2
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object3
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object4
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object5
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object6
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object7
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object8
  $

- Script Parallel Vacuum
  $ cat vac_pll.sh
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object1 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object2 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object3 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object4 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object5 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object6 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object7 &
/usr/local/pgsqlFF/bin/vacuumdb -p 5464 -d db_OBJ -t object8 &
wait
  $


Test 1: Cold Clean database (already previously vacuumed)
=========================================================
 Scenario:
  - stop database
  - flush FS cache (umount/mount)
  - start database
  - execute vacuum script

$ time sh vac_mono.sh
real    4m24.23s
user    0m0.00s
sys     0m0.01s

$ time sh vac_pll.sh
real    1m9.36s
user    0m0.00s
sys     0m0.01s


Test 2: Hot Dirty database (modified and not vacuumed)
======================================================
 Scenario:
  - stop database
  - flush FS cache (umount/mount)
  - start database
  - execute 200.000 updates against each from 8 object' tables
  - execute vacuum script

$ time sh vac_mono.sh
real    9m36.90s
user    0m0.00s
sys     0m0.01s

$ time sh vac_pll.sh
real    2m10.41s
user    0m0.00s
sys     0m0.02s


Speed-up x4 is obtained just because single vacuum process reaching max
80MB/sec in throughput, while with 8 parallel vacuum processes I'm jumping to
360MB/sec... And speakink about Sequential I/O: while you're doing read -
file system may again prefetch incoming data in way once you reclaim next
read - your data will be already in FS cache. However, file system
cannot 'pre-write' data for you - so having more concurrent writers helps a
lot! (Of course in case you have a storage configured to keep concurrent
I/O :))

Well, why all this staff?...
Let's imagine once you need more performance, and you buy 10x times more
performant storage box, will you still able to kill it with a single-process
I/O activity? No... :)  To scale well you need to be able to split your work
in several task executed in parallel. And personally, I'm very happy we can
do it with vacuum now - the one of the most critical part of PostgreSQL...

Best regards!
-Dimitri

From:
Michael Stone
Date:

On Fri, Mar 23, 2007 at 04:37:32PM +0100, Dimitri wrote:
>Speed-up x4 is obtained just because single vacuum process reaching max
>80MB/sec in throughput

I'd look at trying to improve that, it seems very low.

Mike Stone