Re: Is there a significant difference in Memory settings between 9.5 and 12

On Tue, May 12, 2020 at 2:52 PM Tory M Blue <tmblue(at)gmail(dot)com> wrote:
> It took the change but didn't help. So 10GB of shared_buffers in 12 is still a no go. I'm down to 5GB and it works, but this is the same hardware, the same exact 9.5 configuration. So I'm missing something. WE have not had to mess with kernel memory settings since 9.4, so this is an odd one.
>
> I'll keep digging, but i'm hesitant to do my multiple TB db's with half of their shared buffer configs, until I understand what 12 is doing differently than 9.5

Which exact version of 9.5.x are you coming from? What's the exact
error message on 12 (you showed the shared_memory_type=sysv error, but
with the default value (mmap) how does it look)? What's your
huge_pages setting?

Can you reproduce the problem with a freshly created test cluster? As
a regular user, assuming regular RHEL packaging, something like
/usr/pgsql-12/bin/initdb -D test_pgdata, and then
/usr/pgsql-12/bin/postgres -D test_pgdata -c shared_buffers=10GB (then
^C to stop it). If that fails to start in the same way, it'd be
interesting to see the output of the second command with strace in
front of it, in the part where it allocates shared memory. And
perhaps it'd be interesting to see the same output with
/usr/pgsql-9.5/bin/XXX (if you still have the packages). For example,
on my random dev laptop that looks like:

openat(AT_FDCWD, "/proc/meminfo", O_RDONLY) = 6
fstat(6, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
read(6, "MemTotal: 16178852 kB\nMemF"..., 1024) = 1024
read(6, ": 903168 kB\nShmemHugePages: "..., 1024) = 311
close(6) = 0
mmap(NULL, 11016339456, PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_ANONYMOUS|MAP_HUGETLB, -1, 0) = -1 ENOMEM (Cannot
allocate memory)
mmap(NULL, 11016003584, PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7ff74e579000
shmget(0x52e2c1, 56, IPC_CREAT|IPC_EXCL|0600) = 3244038
shmat(3244038, NULL, 0) = 0x7ff9df5ad000

The output is about the same on REL9_5_STABLE and REL_12_STABLE for
me, only slightly different sizes. If that doesn't fail in the same
way on your system with 12, perhaps there are some more settings from
your real clusters required to make it fail. You could add them one
by one with -c foo=bar or in the throw away
test_pgdata/postgresql.conf, and perhaps that process might shed some
light?

I was going to ask if it might be a preloaded extension that is asking
for gobs of extra memory in 12, but we can see from your "Failed
system call was shmget(key=5432001, size=11026235392, 03600)" that
it's in the same ballpark as my total above for shared_buffers=10GB.