> On Jul 14, 2015, at 5:25 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Robert Haas <robertmh...@gmail.com> writes: >> I really think we should do the simple thing first. If we make this >> complicated and add lots of bells and whistles, it is going to be much >> harder to get anything committed, because there will be more things >> for somebody to object to. If we start with something simple, we can >> always improve it later, if we are confident that the design for >> improving it is good. The hardest thing about a proposal like this is >> going to be getting down the overhead to a level that is acceptable, >> and every expansion of the basic design that has been proposed - >> gathering more than one byte of information, or gathering times, or >> having the backend update a tracking hash - adds *significant* >> overhead to the design I proposed. > > FWIW, I entirely share Robert's opinion that adding gettimeofday() > overhead in routinely-taken paths is likely not to be acceptable. > But there's no need to base this solely on opinions. I suggest somebody > try instrumenting just one hotspot in the various ways that are being > proposed, and then we can actually measure what it costs, instead > of guessing about that. > > regards, tom lane >
I made benchmark of gettimeofday(). I believe it is certainly usable for
monitoring.
Testing configuration:
24 cores, Intel Xeon CPU X5675@3.07Ghz
RAM 24 GB
54179703 - microseconds total
2147483647 - (INT_MAX), the number of gettimeofday() calls
>>> 54179703 / 2147483647.0
0.025229390256679331
Here we have the average duration of one gettimeofday in microseconds.
Now we get the count of all waits in one minute (pgbench -i -s 500,
2 GB shared buffers, started with -c 96 -j 4, it is almost 100% cpu load).
b1=# select sum(wait_count) from pg_stat_wait_profile;
sum
---------
2113608
So, we can estimate the time of gtd in one minute (it multiplies by two,
because we are going to call it twice for each wait)
>>> 2113608 * 0.025229390256679331 * 2
106650.08216327897
This is time in microseconds that we spend on gtd in one minute.
Calculation of overhead (in percents):
>>> 106650 / 60000000.0 * 100
0.17775
gtd_test.c
Description: Binary data
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