Also to give perspective on the equivalent writes on CLOG

I used the following script which runs for 10 sec to track all writes to the clog directory and here is what it came up with... (This is with 500 users running)

# cat write.d
#!/usr/sbin/dtrace -s
/execname=="postgres" && dirname(fds[arg0].fi_pathname)=="/export/home0/igen/pgdata/pg_clog"/
   @write[fds[arg0].fi_pathname,arg1] = count();

# ./write.d
dtrace: script './write.d' matched 2 probes
CPU     ID                    FUNCTION:NAME
 3   1026                      :tick-10sec

/export/home0/igen/pgdata/pg_clog/001E -2753028277088 1
I modified read.d to do a 5sec read
# ./read.d
dtrace: script './read.d' matched 3 probes
CPU     ID                    FUNCTION:NAME
 0      1                           :BEGIN
 0   1027                       :tick-5sec

/export/home0/igen/pgdata/pg_clog/001F -2753028268896 1 /export/home0/igen/pgdata/pg_clog/001F -2753028252512 1 /export/home0/igen/pgdata/pg_clog/001F -2753028285280 2 /export/home0/igen/pgdata/pg_clog/001F -2753028277088 3 /export/home0/igen/pgdata/pg_clog/001F -2753028236128 3 /export/home0/igen/pgdata/pg_clog/001E -2753028285280 5 /export/home0/igen/pgdata/pg_clog/001E -2753028236128 9 /export/home0/igen/pgdata/pg_clog/001E -2753028277088 13 /export/home0/igen/pgdata/pg_clog/001E -2753028268896 15 /export/home0/igen/pgdata/pg_clog/001E -2753028252512 27

So the ratio of reads vs writes to clog files is pretty huge..


Jignesh K. Shah wrote:

Here is what I did:

I started aggregating all read information:

First I also had added group by pid (arg0,arg1, pid) and the counts were all coming as 1

Then I just grouped by filename and location (arg0,arg1 of reads) and the counts came back as

# cat read.d
#!/usr/sbin/dtrace -s
   @read[fds[arg0].fi_pathname, arg1] = count();

# ./read.d
dtrace: script './read.d' matched 1 probe

/export/home0/igen/pgdata/pg_clog/0014 -2753028293472 1 /export/home0/igen/pgdata/pg_clog/0014 -2753028277088 1 /export/home0/igen/pgdata/pg_clog/0015 -2753028244320 2 /export/home0/igen/pgdata/pg_clog/0015 -2753028268896 14 /export/home0/igen/pgdata/pg_clog/0015 -2753028260704 25 /export/home0/igen/pgdata/pg_clog/0015 -2753028252512 27 /export/home0/igen/pgdata/pg_clog/0015 -2753028277088 28 /export/home0/igen/pgdata/pg_clog/0015 -2753028293472 37

FYI  I pressed ctrl-c within like less than a second

So to me this seems that multiple processes are reading the same page from different pids. (This was with about 600 suers active.

Aparently we do have a problem that we are reading the same buffer address again. (Same as not being cached anywhere or not finding it in cache anywhere).

I reran lock wait script on couple of processes and did not see CLogControlFileLock as a problem..

# ./83_lwlock_wait.d 14341

            Lock Id            Mode           Count
      WALInsertLock       Exclusive               1
      ProcArrayLock       Exclusive              16

            Lock Id   Combined Time (ns)
      WALInsertLock               383109
      ProcArrayLock            198866236

# ./83_lwlock_wait.d 14607

            Lock Id            Mode           Count
      WALInsertLock       Exclusive               2
      ProcArrayLock       Exclusive              15

            Lock Id   Combined Time (ns)
      WALInsertLock                55243
      ProcArrayLock             69700140


What will help you find out why it is reading the same page again?


Jignesh K. Shah wrote:
I agree with Tom.. somehow I think increasing NUM_CLOG_BUFFERS is just avoiding the symptom to a later value.. I promise to look more into it before making any recommendations to increase NUM_CLOG_BUFFERs.

Because though "iGen" showed improvements in that area by increasing num_clog_buffers , EAStress had shown no improvements.. Plus the reason I think this is not the problem in 8.3beta1 since the Lock Output clearly does not show CLOGControlFile as to be the issue which I had seen in earlier case. So I dont think that increasing NUM_CLOG_BUFFERS will change thing here.

Now I dont understand the code pretty well yet I see three hotspots and not sure if they are related to each other * ProcArrayLock waits - causing Waits as reported by 83_lockwait.d script * SimpleLRUReadPage - causing read IOs as reported by iostat/rsnoop.d
* GetSnapshotData - causing CPU utilization  as reported by hotuser

But I will shut up and do more testing.


Tom Lane wrote:
Josh Berkus <[EMAIL PROTECTED]> writes:
Actually, 32 made a significant difference as I recall ... do you still have the figures for that, Jignesh?

I'd want to see a new set of test runs backing up any call for a change
in NUM_CLOG_BUFFERS --- we've changed enough stuff around this area that
benchmarks using code from a few months back shouldn't carry a lot of

            regards, tom lane

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