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Re: Re: significant performance hit whenever autovacuum runs after upgrading from 9.0 -> 9.1

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On 24/05/12 08:18, Lonni J Friedman wrote:
On Wed, May 23, 2012 at 12:36 PM, Gavin Flower
<GavinFlower@xxxxxxxxxxxxxxxxx> wrote:
On 24/05/12 05:09, Lonni J Friedman wrote:

On Wed, May 23, 2012 at 9:37 AM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:

Lonni J Friedman <netllama@xxxxxxxxx> writes:

After banging my head on the wall for  a long time, I happened to
notice that khugepaged was consuming 100% CPU every time autovacuum
was running.  I did:
echo "madvise" > /sys/kernel/mm/transparent_hugepage/defrag
and immediately the entire problem went away.

Fascinating.

In hindsight, sure.  Before that, it was 2 days of horror.

So this looks like a nasty Fedora16 kernel bug to me, or maybe
postgresql & Fedora16's default kernel settings are just not
compatible?

I agree, kernel bug.  What kernel version are you using exactly?

I'm using the stock 3.3.5-2.fc16.x86_64 kernel that is in Fedora updates.

Is anyone else using Fedora16 & PostgreSQL-9.1 ?

I use an F16 box daily, but can't claim to have done major performance
testing with it.  Can you put together a summary of your nondefault
Postgres settings?  I wonder whether it only kicks in for a certain
size of shared memory for instance.

Oh yea, I'm quite certain that this is somehow related to my setup,
and not a generic problem with all F16/pgsql systems.  For starters,
this problem isn't happening on any of the 3 standby systems, which
are all otherwise identical to the master in every respect.  Also when
we had done some testing (prior to the upgrades), we never ran into
any of these problems.  However our test environment was on smaller
scale hardware, with a much smaller number of clients (and overall
load).

Here are the non default settings in postgresql.conf :
wal_level = hot_standby
archive_mode = on
archive_timeout = 61
max_wal_senders = 10
wal_keep_segments = 5000
hot_standby = on
log_autovacuum_min_duration = 2500
autovacuum_max_workers = 4
maintenance_work_mem = 1GB
checkpoint_completion_target = 0.7
effective_cache_size = 88GB
work_mem = 576MB
wal_buffers = 16MB
checkpoint_segments = 64
shared_buffers = 8GB
max_connections = 350

Let me know if you have any other questions.  I'd be happy to provide
as much information as possible if it can aid in fixing this bug.

I think they will need details of things like: RAM, number/type processors,
number & type
of disks, disk controllers & any other hardware specs that might be relevant
etc.- at very
least: total RAM & number of spindles
16 core Xeon X5550 2.67GHz
128GB RAM
$PGDATA sits on a RAID5 array comprised of 3 SATA disks.  Its Linux's
md software RAID.
How does this compare to your other machines running the same, or similar, databases?
However, you do say that the other machines are indentical - but are the other
machines different in any aspect, that might prove siginificant?


Also anything else running on the box.
nothing else.  its dedicated exclusively to postgresql.

Plus transaction load pattern - over time and read/write ratios.
I'm not sure how I'd obtain this data.  however, the patterns didn't
change since the upgrade.  If someone can point me in the right
direction, I can at least obtain this data as its generated currently.

type/nature of queries
I need some clarification on specifically what you're asking for here.

The complexity, structure, and features of the queries. Do you have lots of sub queries,
and ORDER BY's? Also the number of tables accessed in a query. This is heading into the
territory where others will be better placed to advise you as to what might be relevant!



size of heavily accessed tables and their indexes
there are several rather large tables (90 million+ rows), but most
others are under 1M rows.  However, most tables are accessed & written
to with equal frequency.
The number, type, size, and usage of indexes might be relevant.


I hope the above comments help, probably others will have more specific requests.


Cheers,
Gavin

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