Default shared memory SGML mention
I am looking over the shared memory SGML files, and found that on my
BSD/OS system, I get as the following default shared memory size:
m 131072 5432001 --rw------- postgres user postgres
user 1 1236992 10622 10622 12:11:14 12:11:15 12:11:14
^^^^^^^
I have highlighted the size. If I subtract the default shared memory
buffers, I get:
1236992 - 8192 * 64
712704
so by my computations, we need about 700k added to the shared buffer
cache area. The documentation says to add 512k, not 700k. My guess is
that the non-cached shared memory area has increased in a recent
release, and docs were not updated. I will bump up the SGML mention to
700k unless someone shows a lower default shared memory size.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
Bruce Momjian <pgman@candle.pha.pa.us> writes:
so by my computations, we need about 700k added to the shared buffer
cache area. The documentation says to add 512k, not 700k.
This would depend on your -N setting.
A little experimentation shows that the present multiplier for -B is
about 8350 bytes and the multiplier for -N about 14580 bytes on my
system (HPUX), which extrapolates to a fixed base of about 250K plus
the -B and -N contributions. In reality I believe the computation
is somewhat nonlinear, but that's probably close enough for starters.
regards, tom lane
Bruce Momjian <pgman@candle.pha.pa.us> writes:
so by my computations, we need about 700k added to the shared buffer
cache area. The documentation says to add 512k, not 700k.This would depend on your -N setting.
A little experimentation shows that the present multiplier for -B is
about 8350 bytes and the multiplier for -N about 14580 bytes on my
system (HPUX), which extrapolates to a fixed base of about 250K plus
the -B and -N contributions. In reality I believe the computation
is somewhat nonlinear, but that's probably close enough for starters.
I have updated the SGML sources to say for default shared memory values:
250 kB + 8192 * buffers or infinity
Of course, buffers is 2 * connections by default.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026Bruce Momjian <pgman@candle.pha.pa.us> writes:
A little experimentation shows that the present multiplier for -B is
about 8350 bytes and the multiplier for -N about 14580 bytes on my
system (HPUX), which extrapolates to a fixed base of about 250K plus
the -B and -N contributions. In reality I believe the computation
is somewhat nonlinear, but that's probably close enough for starters.
I have updated the SGML sources to say for default shared memory values:
250 kB + 8192 * buffers or infinity
How did infinity get into this? And what happened to the -N
contribution?
regards, tom lane
Bruce Momjian <pgman@candle.pha.pa.us> writes:
A little experimentation shows that the present multiplier for -B is
about 8350 bytes and the multiplier for -N about 14580 bytes on my
system (HPUX), which extrapolates to a fixed base of about 250K plus
the -B and -N contributions. In reality I believe the computation
is somewhat nonlinear, but that's probably close enough for starters.I have updated the SGML sources to say for default shared memory values:
250 kB + 8192 * buffers or infinity
How did infinity get into this? And what happened to the -N
contribution?
Infinity was always there. The column is labeled "Typical Values". As
far as -N, buffers is always 2*N, right, so I figured it was in there
already.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026Bruce Momjian <pgman@candle.pha.pa.us> writes:
far as -N, buffers is always 2*N, right, so I figured it was in there
already.
No, it isn't, and buffers is NOT always 2*N.
regards, tom lane
Bruce Momjian <pgman@candle.pha.pa.us> writes:
far as -N, buffers is always 2*N, right, so I figured it was in there
already.No, it isn't, and buffers is NOT always 2*N.
Yes, but shared memory limits are based on buffers, not connections,
right? The per-connection overhead is pretty small, no?
If it is buffers and connections, how much is connections and how much
buffers. I know buffers is 8k.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026Bruce Momjian <pgman@candle.pha.pa.us> writes:
Yes, but shared memory limits are based on buffers, not connections,
right? The per-connection overhead is pretty small, no?
I gave you my experimentally-derived formula already:
250Kb + 8350 * B + 14580 * N
I don't see how you can consider the N term negligible.
regards, tom lane
Bruce Momjian <pgman@candle.pha.pa.us> writes:
Yes, but shared memory limits are based on buffers, not connections,
right? The per-connection overhead is pretty small, no?I gave you my experimentally-derived formula already:
250Kb + 8350 * B + 14580 * N
I don't see how you can consider the N term negligible.
Oh, I misread it. I thought the 14.5k was part of the 250k. Let me fix
that.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026Bruce Momjian <pgman@candle.pha.pa.us> writes:
Yes, but shared memory limits are based on buffers, not connections,
right? The per-connection overhead is pretty small, no?I gave you my experimentally-derived formula already:
250Kb + 8350 * B + 14580 * N
I don't see how you can consider the N term negligible.
OK, patch applied.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026