Обсуждение: Memory issues with PostgreSQL 15
Hi all,
We migrated from PostgreSQL 9.4 to PostgreSQL 15 a while ago. Since then, we have a lot of memory issues in our QA
environment(which is a bit tense in resources). We did not have these problems before the migration, and we do not have
themin our production environment, which has a lot more memory. So, it is not super critical for us, but I would still
liketo understand better how we can improve our configuration.
Our PostgreSQL version is "PostgreSQL 15.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat
4.8.5-44),64-bit". The database server is a dedicated server with 15 GB RAM (and 4 cores, if this matters).
We used the following settings:
shared_buffers = 4GB
work_mem = 4MB
After a while, we saw the following error in the logs:
<2024-05-20 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
However, according to "free", a lot of memory was available:
# free -m
total used free shared buff/cache available
Mem: 15882 4992 463 4195 10427 6365
Swap: 1999 271 1728
Our Grafana charts showed a slow increase in memory consumption until it plateaus at 4.66 GB.
We also found the following error:
<2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.2448337832" to
182656bytes: No space left on device
I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately
(aftera few minutes) gave me these errors:
<2024-05-27 11:45:59 CEST - > ERROR: out of memory
<2024-05-27 11:45:59 CEST - > DETAIL: Failed on request of size 201088574 in memory context "TopTransactionContext".
...
<2024-05-27 11:58:02 CEST - > ERROR: out of memory
<2024-05-27 11:58:02 CEST - > DETAIL: Failed while creating memory context "dynahash".
<2024-05-27 11:58:02 CEST - > LOG: background worker "parallel worker" (PID 21480) exited with exit code 1
...
<2024-05-27 12:01:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
<2024-05-27 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
<2024-05-27 12:02:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
Since this seemed worse than before, I changed the setting back to 4 GB. I noticed that "free" now reports even more
availablememory:
# free -m
total used free shared buff/cache available
Mem: 15882 621 320 2256 14940 12674
Swap: 1999 199 1800
So, does the "shared_buffers" setting have the opposite effect than I though? If I correctly remember similar
discussionsyears ago, the database needs both "normal" and shared memory. By increasing the "shared_buffers" to 8 GB, I
mayhave deprived it of "normal" memory. On the other hand, I would have expected the remaining 7 GB to still be
enough.
At this point, I am out of ideas. I clearly seem to misunderstand how the database manages its memory. This may have
changedbetween 9.4 and 15, so my prior knowledge may be useless. I definitely need some help. ☹
Thanks in advance,
Christian
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Hi Christian: On Tue, 28 May 2024 at 18:40, Christian Schröder <christian.schroeder@wsd.com> wrote: > <2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.2448337832" to 182656bytes: No space left on device This hints at some shm function getting an ENOSPC: Coupled with... > I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately(after a few minutes) gave me these errors: A faster fail when increasing it I would start by checking your IPC shared memory limits are ok, especially if you upgraded something in the OS when going from 9 to 15, which seems likely. IIRC in linux you can read them in /proc/sys/kernel/shm*, and they were configured via sysctl. Francisco Olarte.
Hi Francisco, Thank you for your advice. I used "ipcs" to get more readable information about the shared memory: # ipcs -m -l --human ------ Shared Memory Limits -------- max number of segments = 4096 max seg size = 16E max total shared memory = 16E min seg size = 1B # ipcs -m ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x04000194 35 postgres 600 56 19 # ipcs -m -i 35 Shared memory Segment shmid=35 uid=26 gid=26 cuid=26 cgid=26 mode=0600 access_perms=0600 bytes=56 lpid=7653 cpid=3875 nattch=19 att_time=Tue May 28 22:56:35 2024 det_time=Tue May 28 22:56:35 2024 change_time=Tue May 28 07:59:59 2024 As far as I understand, there is no upper limit to the size of the shared memory. The database only holds a single sharedmemory segment, which doesn't seem to have a relevant size. I am surprised to see this since I would have expected much more shared memory to be used by the database. Is there anythingin the configuration that prevents the shared memory from being used? Best, Christian -----Original Message----- From: Francisco Olarte <folarte@peoplecall.com> Sent: Tuesday, May 28, 2024 7:15 PM To: Christian Schröder <christian.schroeder@wsd.com> Cc: pgsql-general@lists.postgresql.org; Eric Wong <eric.wong@wsd.com> Subject: Re: Memory issues with PostgreSQL 15 [EXTERNAL] Hi Christian: On Tue, 28 May 2024 at 18:40, Christian Schröder <christian.schroeder@wsd.com> wrote: > <2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize > shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No > space left on device This hints at some shm function getting an ENOSPC: Coupled with... > I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately(after a few minutes) gave me these errors: A faster fail when increasing it I would start by checking your IPC shared memory limits are ok, especially if you upgradedsomething in the OS when going from 9 to 15, which seems likely. IIRC in linux you can read them in /proc/sys/kernel/shm*, and they were configured via sysctl. Francisco Olarte. ---------------------------------------------- SUPPORT: For any issues, inquiries, or assistance, please contact our support team at support@wsd.com. Our dedicated team is availableto help you and provide prompt assistance. CONFIDENTIALITY NOTICE: This email and any attachments are confidential and intended solely for the use of the individual or entity to whom it isaddressed. If you have received this email in error, please notify the sender immediately and delete it from your system.
Hi Christian: On Wed, 29 May 2024 at 00:59, Christian Schröder <christian.schroeder@wsd.com> wrote: > Thank you for your advice. I used "ipcs" to get more readable information about the shared memory: ... > As far as I understand, there is no upper limit to the size of the shared memory. The database only holds a single sharedmemory segment, which doesn't seem to have a relevant size. Seems the same to me, so I will disregard that. > I am surprised to see this since I would have expected much more shared memory to be used by the database. Is there anythingin the configuration that prevents the shared memory from being used? I am not too current with postgres, that one was a thing which happened to me when I did more administration, and is one you always want to check. I assume you have all checked, but I would follow by insuring every mounted partition in your system has space. I am not current on the details, but I know Pg can be mmaping things and doing other stuff. Your problem seems more of resource exhaustion, so I would follow by checking that, both disk, tmpfs and all the other stuff. I cannot give you advice on that as it depends a lot on your server configuration and from the age in the mssages I suspect you have the usual suspects debugged. But as you have a configuration crahsing in minutes and it seems to be a dev server you could do it easily. Sorry. Francisco Olarte.
Hi all,
We migrated from PostgreSQL 9.4 to PostgreSQL 15 a while ago. Since then, we have a lot of memory issues in our QA environment (which is a bit tense in resources). We did not have these problems before the migration, and we do not have them in our production environment, which has a lot more memory. So, it is not super critical for us, but I would still like to understand better how we can improve our configuration.
Our PostgreSQL version is "PostgreSQL 15.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit". The database server is a dedicated server with 15 GB RAM (and 4 cores, if this matters).
We used the following settings:
shared_buffers = 4GB
work_mem = 4MB
After a while, we saw the following error in the logs:
<2024-05-20 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
However, according to "free", a lot of memory was available:
# free -m
total used free shared buff/cache available
Mem: 15882 4992 463 4195 10427 6365
Swap: 1999 271 1728
Our Grafana charts showed a slow increase in memory consumption until it plateaus at 4.66 GB.
We also found the following error:
<2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No space left on device
I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately (after a few minutes) gave me these errors:
<2024-05-27 11:45:59 CEST - > ERROR: out of memory
<2024-05-27 11:45:59 CEST - > DETAIL: Failed on request of size 201088574 in memory context "TopTransactionContext".
...
<2024-05-27 11:58:02 CEST - > ERROR: out of memory
<2024-05-27 11:58:02 CEST - > DETAIL: Failed while creating memory context "dynahash".
<2024-05-27 11:58:02 CEST - > LOG: background worker "parallel worker" (PID 21480) exited with exit code 1
...
<2024-05-27 12:01:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
<2024-05-27 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
<2024-05-27 12:02:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
Since this seemed worse than before, I changed the setting back to 4 GB. I noticed that "free" now reports even more available memory:
# free -m
total used free shared buff/cache available
Mem: 15882 621 320 2256 14940 12674
Swap: 1999 199 1800
So, does the "shared_buffers" setting have the opposite effect than I though? If I correctly remember similar discussions years ago, the database needs both "normal" and shared memory. By increasing the "shared_buffers" to 8 GB, I may have deprived it of "normal" memory. On the other hand, I would have expected the remaining 7 GB to still be enough.
At this point, I am out of ideas. I clearly seem to misunderstand how the database manages its memory. This may have changed between 9.4 and 15, so my prior knowledge may be useless. I definitely need some help. ☹
Thanks in advance,
Christian
----------------------------------------------
SUPPORT:
For any issues, inquiries, or assistance, please contact our support team at support@wsd.com. Our dedicated team is available to help you and provide prompt assistance.
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=?utf-8?B?Q2hyaXN0aWFuIFNjaHLDtmRlcg==?= <christian.schroeder@wsd.com> writes:
> # ipcs -m
> ------ Shared Memory Segments --------
> key shmid owner perms bytes nattch status
> 0x04000194 35 postgres 600 56 19
> I am surprised to see this since I would have expected much more shared memory to be used by the database. Is there
anythingin the configuration that prevents the shared memory from being used?
SysV shared memory isn't that relevant to Postgres anymore. Most
of what we allocate goes into POSIX-style shared memory segments,
which are not shown by "ipcs". We do still create one small
fixed-size data structure in SysV memory, which is what you're
seeing here, for arcane reasons having to do with the lifespan of
the shared memory segments being different in those two APIs.
>> <2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize
>> shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No
>> space left on device
This seems to indicate that you're hitting some kernel limit on
the amount of POSIX shared memory. Not sure where to look for
that.
regards, tom lane
Il giorno mar 28 mag 2024 alle ore 18:40 Christian Schröder <christian.schroeder@wsd.com> ha scritto: > Our PostgreSQL version is "PostgreSQL 15.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44),64-bit". The database server is a dedicated server with 15 GB RAM (and 4 cores, if this matters). Maybe you have PostgreSQL running inside a container with capped resources (I saw this on some recent distro, running it from systemd)?
Hi Francisco, Unfortunately, all disks have plenty of free space, so this can be ruled out as a reason. I will follow up on the other suggestions from the list. Best, Christian -----Original Message----- From: Francisco Olarte <folarte@peoplecall.com> Sent: Wednesday, May 29, 2024 10:24 AM To: Christian Schröder <christian.schroeder@wsd.com> Cc: pgsql-general@lists.postgresql.org; Eric Wong <eric.wong@wsd.com> Subject: Re: Memory issues with PostgreSQL 15 [EXTERNAL] Hi Christian: On Wed, 29 May 2024 at 00:59, Christian Schröder <christian.schroeder@wsd.com> wrote: > Thank you for your advice. I used "ipcs" to get more readable information about the shared memory: ... > As far as I understand, there is no upper limit to the size of the shared memory. The database only holds a single sharedmemory segment, which doesn't seem to have a relevant size. Seems the same to me, so I will disregard that. > I am surprised to see this since I would have expected much more shared memory to be used by the database. Is there anythingin the configuration that prevents the shared memory from being used? I am not too current with postgres, that one was a thing which happened to me when I did more administration, and is oneyou always want to check. I assume you have all checked, but I would follow by insuring every mounted partition in yoursystem has space. I am not current on the details, but I know Pg can be mmaping things and doing other stuff. Your problemseems more of resource exhaustion, so I would follow by checking that, both disk, tmpfs and all the other stuff. Icannot give you advice on that as it depends a lot on your server configuration and from the age in the mssages I suspectyou have the usual suspects debugged. But as you have a configuration crahsing in minutes and it seems to be a devserver you could do it easily. Sorry. Francisco Olarte. ---------------------------------------------- SUPPORT: For any issues, inquiries, or assistance, please contact our support team at support@wsd.com. Our dedicated team is availableto help you and provide prompt assistance. CONFIDENTIALITY NOTICE: This email and any attachments are confidential and intended solely for the use of the individual or entity to whom it isaddressed. If you have received this email in error, please notify the sender immediately and delete it from your system.
Hi Salahuddin,
I had already checked most of your points, but I double checked them now.
# free -m
total used free shared buff/cache available
Mem: 15882 1466 269 2110 14147 11976
Swap: 1999 254 1745
Free memory seems to be low, which is normal because most of the memory is used by buffers and caches. As you can see, the available memory is almost 12 GB.
shared_buffers, work_mem, etc.
Our initial setting for “shared_buffers” was 4 GB, which is roughly 25% of the system memory; however, I tried different values (see my original message), but none of them seemed to work. We also played around with the other settings but couldn’t find any combination that worked.
Shared memory limits look good to me:
# sudo sysctl -a | grep kernel.shm
kernel.shmall = 18446744073692774399
kernel.shmmax = 18446744073692774399
kernel.shmmni = 4096
Thanks,
Christian
From: Muhammad Salahuddin Manzoor <salahuddin.m@bitnine.net>
Sent: Wednesday, May 29, 2024 11:41 AM
To: Christian Schröder <christian.schroeder@wsd.com>
Cc: pgsql-general <pgsql-general@lists.postgresql.org>; Eric Wong <eric.wong@wsd.com>
Subject: Re: Memory issues with PostgreSQL 15
[EXTERNAL]
Greetings,
The error message you encountered, "could not fork autovacuum worker process: Cannot allocate memory," indicates that your PostgreSQL server attempted to start an autovacuum worker process but failed because the system ran out of memory.
Steps to verify.
1 Check system available memory with commands.
free -m
top
2. Check PG configurations.
shared_buffers --Typically 25% of total mem.
work_mem
maintenance_work_mem--For maintenance op like autovaccume create index etc. Increase it to 64MB or appropriate to your requirement.
max_connections
Monitor /var/log/messages file for errors.
2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No space left on device
Check share memory limits.
/etc/sysctl.conf
kernel.shmmax = 68719476736 # Example value, adjust as needed
kernel.shmall = 16777216 # Example value, adjust as needed
Restart system and db
Ensure you have enough disk space available check and monitor disk space with command
df -h
Reduce max_parallel_workers_per_gather = 2;
If it is set to high value.
I think setting up OS parameter.
Increasing maintenance mem value and reducing max paralell workers xan help in solution.
Regards,
Salahuddin.
On Tue, 28 May 2024, 21:40 Christian Schröder, <christian.schroeder@wsd.com> wrote:
Hi all,
We migrated from PostgreSQL 9.4 to PostgreSQL 15 a while ago. Since then, we have a lot of memory issues in our QA environment (which is a bit tense in resources). We did not have these problems before the migration, and we do not have them in our production environment, which has a lot more memory. So, it is not super critical for us, but I would still like to understand better how we can improve our configuration.
Our PostgreSQL version is "PostgreSQL 15.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit". The database server is a dedicated server with 15 GB RAM (and 4 cores, if this matters).
We used the following settings:
shared_buffers = 4GB
work_mem = 4MB
After a while, we saw the following error in the logs:
<2024-05-20 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
However, according to "free", a lot of memory was available:
# free -m
total used free shared buff/cache available
Mem: 15882 4992 463 4195 10427 6365
Swap: 1999 271 1728
Our Grafana charts showed a slow increase in memory consumption until it plateaus at 4.66 GB.
We also found the following error:
<2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No space left on device
I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately (after a few minutes) gave me these errors:
<2024-05-27 11:45:59 CEST - > ERROR: out of memory
<2024-05-27 11:45:59 CEST - > DETAIL: Failed on request of size 201088574 in memory context "TopTransactionContext".
...
<2024-05-27 11:58:02 CEST - > ERROR: out of memory
<2024-05-27 11:58:02 CEST - > DETAIL: Failed while creating memory context "dynahash".
<2024-05-27 11:58:02 CEST - > LOG: background worker "parallel worker" (PID 21480) exited with exit code 1
...
<2024-05-27 12:01:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
<2024-05-27 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
<2024-05-27 12:02:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
Since this seemed worse than before, I changed the setting back to 4 GB. I noticed that "free" now reports even more available memory:
# free -m
total used free shared buff/cache available
Mem: 15882 621 320 2256 14940 12674
Swap: 1999 199 1800
So, does the "shared_buffers" setting have the opposite effect than I though? If I correctly remember similar discussions years ago, the database needs both "normal" and shared memory. By increasing the "shared_buffers" to 8 GB, I may have deprived it of "normal" memory. On the other hand, I would have expected the remaining 7 GB to still be enough.
At this point, I am out of ideas. I clearly seem to misunderstand how the database manages its memory. This may have changed between 9.4 and 15, so my prior knowledge may be useless. I definitely need some help. ☹
Thanks in advance,
Christian
----------------------------------------------
SUPPORT:
For any issues, inquiries, or assistance, please contact our support team at support@wsd.com. Our dedicated team is available to help you and provide prompt assistance.
CONFIDENTIALITY NOTICE:
This email and any attachments are confidential and intended solely for the use of the individual or entity to whom it is addressed. If you have received this email in error, please notify the sender immediately and delete it from your system.
----------------------------------------------
SUPPORT:
For any issues, inquiries, or assistance, please contact our support team at support@wsd.com. Our dedicated team is available to help you and provide prompt assistance.
CONFIDENTIALITY NOTICE:
This email and any attachments are confidential and intended solely for the use of the individual or entity to whom it is addressed. If you have received this email in error, please notify the sender immediately and delete it from your system.
On Thu, 30 May 2024 at 09:37, Christian Schröder <christian.schroeder@wsd.com> wrote: > Unfortunately, all disks have plenty of free space, so this can be ruled out as a reason. > I will follow up on the other suggestions from the list. Do not forget to check all mounted filesystems, not only disks. Specially /dev/shm, IIRC its mounted size is one limit for posix shared memory. Francisco Olarte.
Hi christian: On Thu, 30 May 2024 at 12:51, Christian Schröder <christian.schroeder@wsd.com> wrote: ... > I had already checked most of your points, but I double checked them now. ... > Shared memory limits look good to me: > # sudo sysctl -a | grep kernel.shm > kernel.shmall = 18446744073692774399 > kernel.shmmax = 18446744073692774399 > kernel.shmmni = 4096 Bear in mind this is SysV shared memory. IIRC Pg uses POSIX shared memory for shared buffers, which I think is backed normally in Linux by files in a tmpfs mounted on /dev/shm. It still uses some amount of SysV due to some special properties lacking from POSIX, for control purposes, but only a little. You could try "df -h /dev/shm" and "ls -lhR /dev/shm/" to see if you have problems there. Francisco Olarte.