PostgreSQL's share_buffer calculation using shmget() versus kernel.shmmax

[Mel Llaguno]

All,

I'm wondering about how postgresql calculates the value for shared buffers as I see some discrepancies with what the following script provides versus what is recommended in the pgctl.log when the database fails to start.

#!/bin/bash
# simple shmsetup script
page_size=`getconf PAGE_SIZE`
phys_pages=`getconf _PHYS_PAGES`
shmall=`expr $phys_pages / 2`
shmmax=`expr $shmall \* $page_size`
echo kernel.shmmax = $shmmax
echo kernel.shmall = $shmall

Any pointers which would explain these differences would be greatly appreciate. Ultimately, I'd like to calculate the expected kernel.shmmax which matches postgresql's shmget() call.

TIA,

Mel

[Scott Ribe]

Post by Mel Llaguno
Any pointers which would explain these differences would be greatly appreciate.

Postgres is likely not the only thing on your system that allocates shared memory.
--
Scott Ribe
***@elevated-dev.com
http://www.elevated-dev.com/
[phone] voice

[K P Manoj]

Hi ,

You are mentioned SHMMAX larger value is no harm for the database , can i
keep this value as 100% of RAM ?

Right now we have two cluster in this server , one is having 8 GB and other
2 GB shared buffer .
But i am facing some issue , OS cache is filled frequently once i run some
query on database its uses 100 % of the processor also I am unable to
login the database.

Also query is taking more time as normal, seems to be I/O as normal.


DETAILS
========

kernel.shmmax = 68719476736

kernel.shmall = 4294967296


[***@xxxx ~]$ free -m
total used free shared buffers cached
Mem: 64433 48750 15682 0 240 38327
-/+ buffers/cache: 10182 54250
Swap: 6027 0 6027

Tom,
Thanks for the response. I've been doing a lot of performance tuning for
our customers and I've found that wiki link a life saver ;-)
I'm trying to come up with a precise way to calculate the shmget() value
which postgresql uses in the pgctl.log message when the kernel.shmmax is
set too low. There are situations when knowing this exact value is useful
as our customers are sometimes not as familiar with postgresql as we'd
like. Being able to calculate this value from enabled settings in
postgresql.conf would help us provide accurate guidance. As per Pavan's
suggestion, I'm having a look at the src/backend/storage/ipc/ipci.c.
Thanks,
Mel
________________________________________
Sent: Wednesday, February 06, 2013 10:49 PM
To: Mel Llaguno
Subject: Re: [ADMIN] PostgreSQL's share_buffer calculation using shmget()
versus kernel.shmmax

Thanks for your reply. I agree with your statement that you should set

the configuration parameters first, but I would like to be able to
calculate the SHMMAX value based on those parameters. This is particularly
useful when suggesting postgresql.conf optimizations to our customers whose
machine have a lot of RAM (64+GB). Having to guess this value is far from
ideal; what I'd like is the formula used by postgresql that generates the
shmget() value displayed in the pgctl.log.
http://www.postgresql.org/docs/9.2/static/kernel-resources.html#SYSVIPC
As Pavan says, the shared_buffers term is usually the only one worth
worrying about. The traditional advice is to not set that to more than
about a quarter of your physical RAM, which would mean that this script
you're using to set SHMMAX is leaving lots of headroom, which is
perfectly OK. (AFAIK there is no penalty to setting SHMMAX larger than
you need.)
https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
regards, tom lane
--
http://www.postgresql.org/mailpref/pgsql-admin

[Mel Llaguno]

Tom,

Thanks for the response. I've been doing a lot of performance tuning for our customers and I've found that wiki link a life saver ;-)

I'm trying to come up with a precise way to calculate the shmget() value which postgresql uses in the pgctl.log message when the kernel.shmmax is set too low. There are situations when knowing this exact value is useful as our customers are sometimes not as familiar with postgresql as we'd like. Being able to calculate this value from enabled settings in postgresql.conf would help us provide accurate guidance. As per Pavan's suggestion, I'm having a look at the src/backend/storage/ipc/ipci.c.

Thanks,

Mel
________________________________________
From: Tom Lane [***@sss.pgh.pa.us]
Sent: Wednesday, February 06, 2013 10:49 PM
To: Mel Llaguno
Cc: Pavan Deolasee; pgsql-***@postgresql.org
Subject: Re: [ADMIN] PostgreSQL's share_buffer calculation using shmget() versus kernel.shmmax

Thanks for your reply. I agree with your statement that you should set the configuration parameters first, but I would like to be able to calculate the SHMMAX value based on those parameters. This is particularly useful when suggesting postgresql.conf optimizations to our customers whose machine have a lot of RAM (64+GB). Having to guess this value is far from ideal; what I'd like is the formula used by postgresql that generates the shmget() value displayed in the pgctl.log.

There's some rather old information in Table 17-2 here:
http://www.postgresql.org/docs/9.2/static/kernel-resources.html#SYSVIPC

As Pavan says, the shared_buffers term is usually the only one worth
worrying about. The traditional advice is to not set that to more than
about a quarter of your physical RAM, which would mean that this script
you're using to set SHMMAX is leaving lots of headroom, which is
perfectly OK. (AFAIK there is no penalty to setting SHMMAX larger than
you need.)

There's more info worth looking at here:
https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server

regards, tom lane

[Tom Lane]

Thanks for your reply. I agree with your statement that you should set the configuration parameters first, but I would like to be able to calculate the SHMMAX value based on those parameters. This is particularly useful when suggesting postgresql.conf optimizations to our customers whose machine have a lot of RAM (64+GB). Having to guess this value is far from ideal; what I'd like is the formula used by postgresql that generates the shmget() value displayed in the pgctl.log.

There's some rather old information in Table 17-2 here:
http://www.postgresql.org/docs/9.2/static/kernel-resources.html#SYSVIPC

As Pavan says, the shared_buffers term is usually the only one worth
worrying about. The traditional advice is to not set that to more than
about a quarter of your physical RAM, which would mean that this script
you're using to set SHMMAX is leaving lots of headroom, which is
perfectly OK. (AFAIK there is no penalty to setting SHMMAX larger than
you need.)

There's more info worth looking at here:
https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server

regards, tom lane

[Mel Llaguno]

Pavan,

Thanks. I'll have a look at the source code.

M.
________________________________________
From: Pavan Deolasee [***@gmail.com]
Sent: Wednesday, February 06, 2013 10:41 PM
To: Mel Llaguno
Cc: pgsql-***@postgresql.org
Subject: Re: [ADMIN] PostgreSQL's share_buffer calculation using shmget() versus kernel.shmmax

Having to guess this value is far from ideal; what I'd like is the formula used by postgresql that generates the shmget() value displayed in the pgctl.log.

There is no easy way or at least none that I'm aware of, to get the
exact value of shared memory needed for Postgres. If you have access
to the source code, you can look at CreateSharedMemoryAndSemaphores()
function in src/backend/storage/ipc/ipci.c to see what all goes in
determining the size for shmget()

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee

[Pavan Deolasee]

Having to guess this value is far from ideal; what I'd like is the formula used by postgresql that generates the shmget() value displayed in the pgctl.log.

There is no easy way or at least none that I'm aware of, to get the
exact value of shared memory needed for Postgres. If you have access
to the source code, you can look at CreateSharedMemoryAndSemaphores()
function in src/backend/storage/ipc/ipci.c to see what all goes in
determining the size for shmget()

Thanks,
Pavan

[Mel Llaguno]

Pavan,

Thanks for your reply. I agree with your statement that you should set the configuration parameters first, but I would like to be able to calculate the SHMMAX value based on those parameters. This is particularly useful when suggesting postgresql.conf optimizations to our customers whose machine have a lot of RAM (64+GB). Having to guess this value is far from ideal; what I'd like is the formula used by postgresql that generates the shmget() value displayed in the pgctl.log.

Thanks,

Mel
________________________________________
From: Pavan Deolasee [***@gmail.com]
Sent: Wednesday, February 06, 2013 10:12 PM
To: Mel Llaguno
Cc: pgsql-***@postgresql.org
Subject: Re: [ADMIN] PostgreSQL's share_buffer calculation using shmget() versus kernel.shmmax

Post by Mel Llaguno
All,
I'm wondering about how postgresql calculates the value for shared buffers
as I see some discrepancies with what the following script provides versus
what is recommended in the pgctl.log when the database fails to start.
#!/bin/bash
# simple shmsetup script
page_size=`getconf PAGE_SIZE`
phys_pages=`getconf _PHYS_PAGES`
shmall=`expr $phys_pages / 2`
shmmax=`expr $shmall \* $page_size`
echo kernel.shmmax = $shmmax
echo kernel.shmall = $shmall
Any pointers which would explain these differences would be greatly
appreciate. Ultimately, I'd like to calculate the expected kernel.shmmax
which matches postgresql's shmget() call.

I don't know where you got hold of this script, but it seems this is
setting SHMMAX equal to half the size of RAM. So if your system has
4GB RAM, a process can request maximum of 2GB of shared memory. The
amount of shared memory PostgreSQL needs is governed by several
configuration parameters, but the most important and the one which
will eat up most of that shared memory is "shared_buffers". So if you
want to work within the bounds of SHMMAX set by this script, you would
need to set shared_buffers a notch lower than that. But often its
easier and better to decide your Postgres configuration parameters and
then set SHMMAX to satisfy that.

Thanks,
Pavan


--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee

[Pavan Deolasee]

Post by Mel Llaguno
All,
I'm wondering about how postgresql calculates the value for shared buffers
as I see some discrepancies with what the following script provides versus
what is recommended in the pgctl.log when the database fails to start.
#!/bin/bash
# simple shmsetup script
page_size=`getconf PAGE_SIZE`
phys_pages=`getconf _PHYS_PAGES`
shmall=`expr $phys_pages / 2`
shmmax=`expr $shmall \* $page_size`
echo kernel.shmmax = $shmmax
echo kernel.shmall = $shmall
Any pointers which would explain these differences would be greatly
appreciate. Ultimately, I'd like to calculate the expected kernel.shmmax
which matches postgresql's shmget() call.

I don't know where you got hold of this script, but it seems this is
setting SHMMAX equal to half the size of RAM. So if your system has
4GB RAM, a process can request maximum of 2GB of shared memory. The
amount of shared memory PostgreSQL needs is governed by several
configuration parameters, but the most important and the one which
will eat up most of that shared memory is "shared_buffers". So if you
want to work within the bounds of SHMMAX set by this script, you would
need to set shared_buffers a notch lower than that. But often its
easier and better to decide your Postgres configuration parameters and
then set SHMMAX to satisfy that.

Thanks,
Pavan