Re: Shared memory usage calculations

2009/10/10 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>

>
> There is no "default value", we are talking about the actual size of an
> actual memory object. Which you find out with ipcs, not sysctl.
> For instance
>
> $ ipcs -a
>
> ------ Shared Memory Segments --------
> key shmid owner perms bytes nattch status
> 0x00530201 3080195 tgl 600 37904384 4
>
> ------ Semaphore Arrays --------
> key semid owner perms nsems
> 0x00530201 20873216 tgl 600 17
> 0x00530202 20905985 tgl 600 17
> 0x00530203 20938754 tgl 600 17
> 0x00530204 20971523 tgl 600 17
> 0x00530205 21004292 tgl 600 17
> 0x00530206 21037061 tgl 600 17
> 0x00530207 21069830 tgl 600 17
>
> ------ Message Queues --------
> key msqid owner perms used-bytes messages
>
> (I'm running this postmaster as "tgl", not the more usual "postgres".
> On some platforms you might need to be the postgres user or root to see
> shared objects belonging to postgres.)
>
> regards, tom lane
>

My mistake. I've never seen that before. Running it on mine returns the
following:

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status

0x0052e2c1 491520 postgres 600 235511808 4

------ Semaphore Arrays --------
key semid owner perms nsems
0x0052e2c1 8585218 postgres 600 17
0x0052e2c2 8617987 postgres 600 17
0x0052e2c3 8650756 postgres 600 17
0x0052e2c4 8683525 postgres 600 17
0x0052e2c5 8716294 postgres 600 17
0x0052e2c6 8749063 postgres 600 17
0x0052e2c7 8781832 postgres 600 17
0x0052e2c8 8814601 postgres 600 17
0x0052e2c9 8847370 postgres 600 17
0x0052e2ca 8880139 postgres 600 17
0x0052e2cb 8912908 postgres 600 17
0x0052e2cc 8945677 postgres 600 17
0x0052e2cd 8978446 postgres 600 17
0x0052e2ce 9011215 postgres 600 17
0x0052e2cf 9043984 postgres 600 17
0x0052e2d0 9076753 postgres 600 17
0x0052e2d1 9109522 postgres 600 17
0x0052e2d2 9142291 postgres 600 17
0x0052e2d3 9175060 postgres 600 17
0x0052e2d4 9207829 postgres 600 17
0x0052e2d5 9240598 postgres 600 17
0x0052e2d6 9273367 postgres 600 17
0x0052e2d7 9306136 postgres 600 17
0x0052e2d8 9338905 postgres 600 17
0x0052e2d9 9371674 postgres 600 17
0x0052e2da 9404443 postgres 600 17
0x0052e2db 9437212 postgres 600 17
0x0052e2dc 9469981 postgres 600 17
0x0052e2dd 9502750 postgres 600 17
0x0052e2de 9535519 postgres 600 17
0x0052e2df 9568288 postgres 600 17
0x0052e2e0 9601057 postgres 600 17
0x0052e2e1 9633826 postgres 600 17
0x0052e2e2 9666595 postgres 600 17
0x0052e2e3 9699364 postgres 600 17
0x0052e2e4 9732133 postgres 600 17
0x0052e2e5 9764902 postgres 600 17
0x0052e2e6 9797671 postgres 600 17
0x0052e2e7 9830440 postgres 600 17
0x0052e2e8 9863209 postgres 600 17
0x0052e2e9 9895978 postgres 600 17
0x0052e2ea 9928747 postgres 600 17
0x0052e2eb 9961516 postgres 600 17
0x0052e2ec 9994285 postgres 600 17
0x0052e2ed 10027054 postgres 600 17
0x0052e2ee 10059823 postgres 600 17
0x0052e2ef 10092592 postgres 600 17
0x0052e2f0 10125361 postgres 600 17
0x0052e2f1 10158130 postgres 600 17
0x0052e2f2 10190899 postgres 600 17
0x0052e2f3 10223668 postgres 600 17
0x0052e2f4 10256437 postgres 600 17
0x0052e2f5 10289206 postgres 600 17
0x0052e2f6 10321975 postgres 600 17
0x0052e2f7 10354744 postgres 600 17
0x0052e2f8 10387513 postgres 600 17
0x0052e2f9 10420282 postgres 600 17
0x0052e2fa 10453051 postgres 600 17
0x0052e2fb 10485820 postgres 600 17
0x0052e2fc 10518589 postgres 600 17

------ Message Queues --------
key msqid owner perms used-bytes messages

In any case, I gather that the documentation isn't inaccurate, rather my
interpretation is. And having put my number of connections right up to
1500, Postgres still starts fine, so I'm not sure why I thought it would
cause the shared memory limit to be reached so easily.

Thanks

Thom