I changed CLOG Buffers to 16
Running the test again:
# ./read.d
dtrace: script './read.d' matched 2 probes
CPU ID FUNCTION:NAME
0 1027 :tick-5sec
/export/home0/igen/pgdata/pg_clog/0024
-2753028219296 1
/export/home0/igen/pgdata/pg_clog/0025
-2753028211104 1
# ./read.d
dtrace: script './read.d' matched 2 probes
CPU ID FUNCTION:NAME
1 1027 :tick-5sec
# ./read.d
dtrace: script './read.d' matched 2 probes
CPU ID FUNCTION:NAME
1 1027 :tick-5sec
# ./read.d
dtrace: script './read.d' matched 2 probes
CPU ID FUNCTION:NAME
0 1027 :tick-5sec
/export/home0/igen/pgdata/pg_clog/0025
-2753028194720 1
So Tom seems to be correct that it is a case of CLOG Buffer thrashing.
But since I saw the same problem with two different workloads, I think
people hitting this problem is pretty high.
Also I am bit surprised that CLogControlFile did not show up as being
hot.. Maybe because not much writes are going on .. Or maybe since I did
not trace all 500 users to see their hot lock status..
Dmitri has another workload to test, I might try that out later on to
see if it causes similar impact or not.
Of course I havent seen my throughput go up yet since I am already CPU
bound... But this is good since the number of IOPS to the disk are
reduced (and hence system calls).
If I take this as my baseline number.. I can then proceed to hunt other
bottlenecks????
Whats the view of the community?
Hunt down CPU utilizations or Lock waits next?
Your votes are crucial on where I put my focus.
Another thing Josh B told me to check out was the wal_writer_delay setting:
I have done two settings with almost equal performance (with the CLOG 16
setting) .. One with 100ms and other default at 200ms.. Based on the
runs it seemed that the 100ms was slightly better than the default ..
(Plus the risk of loosing data is reduced from 600ms to 300ms)
Thanks.
Regards,
Jignesh
Tom Lane wrote:
"Jignesh K. Shah" <J.K.Shah@xxxxxxx> writes:
So the ratio of reads vs writes to clog files is pretty huge..
It looks to me that the issue is simply one of not having quite enough
CLOG buffers. Your first run shows 8 different pages being fetched and
the second shows 10. Bearing in mind that we "pin" the latest CLOG page
into buffers, there are only NUM_CLOG_BUFFERS-1 buffers available for
older pages, so what we've got here is thrashing for the available
slots.
Try increasing NUM_CLOG_BUFFERS to 16 and see how it affects this test.
regards, tom lane
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