From the latest AWR - SQL ordered by, Segments by, Buffer pool ...
General guideline - you won't beat the LRU algorithms for the buffer pool,
so assigning a keep pool is usually counter productive. Sometimes it's
worth creating a relativbely small RECYCLE pool to stop some large object
"polluting" the default buffer pool with blocks that are never re-used once
read (in your case you don't seem to be using it at all in this interval,
so if you find no objects assigned to it then you might as well get rid of
that too and assign the memory to the default.
For a guideline on how much bigger to make the buffer pool, there the
buffer cache advisory - check to see what Oracle thinks will make a big
difference - it's not a good guideline but it might give you some idea as
to wether (e.g. doubling it) will help or whether you need to go to
something much larger (like 10 times - and given that you've apparanetly
got about 250GB of free memory in the machine maybe even 10 times would be
viable). The uncertainty is made greater because you have to consider the
effect of getting rid of the keep and recycle. You should be able to
resize dynamically (within the 24GB total you've got), though, so at least
you don't have to restart the instance to change the pools.
You might also look at v$sgastat (there's a section in the report about the
library cache detail) for memory in the shared pool labelled something
likeL "KGH: no access" - this is shared pool memory being used to hold data
blocks - but I don't think you'll see any because I suspect you don't have
the sga_target set and are depending on fixed memory sizes.
The Executions tell you where the application problem is: it's not just
that there are very big numbers of single row executions - the 3 "new"
statements in the busy period point you to part of the application. So you
can find out what (business) is happening there, and find out if there are
multiple concurrent copies of the same (business) process running - you may
find that if you can reduce the number of copies of that process then the
database will survive the experience rather better. It does look as if
whatever it is it's somehow connected with the streams processing -- but
that may simply be "we do more X, and streams is used to copy X to
somewhere else".
You'll notice, of course, that two of the three big new statements are an
insert and an update - they are presumably the source of all the extra db
block changes, and the large number of writes (and reads). For a pure "fix
the SQL not the problem" approach you could check the indexing on the
target tables to see if there are any redundant indexes adding to the
workload; similarly for the other two high-usage statements you could check
that the select statements are using the most efficient path or whether the
path could be made more efficient.
Bottom line - for this 15 minute each day (if that's really all it is) your
machine is simply overloaded by the amount of work it's being told to do.
You seem to be protected on physical reads by a massive file system cache,
but the cache doesn't protect you on writes so (even in the "normal"
period) your writes are slow and when you go under heavy load the write
response becomes catastrophic resulting in demands for free buffers that
can't be met.
Regards
Jonathan Lewis
On Mon, 18 Jan 2021 at 11:40, Pap <oracle.developer35@xxxxxxxxx> wrote:
Thank You Jonathan.
My mistake, I didn't notice the KEEP and other pools allocation of buffer
cache were hardcoded.
As per attached the AWR section from buffer pool statistics and streams
activity. Buffer pool statistics does show , it's the default buffer cache
which is experiencing all the "free buffer waits". Zero "free buffer waits"
logged against Keep and recycle pool. Then moving down to segment by block
changes , I see two of the table partitions + index partition at the top.
And the "sql order by execution" does map to the sqls doing row by row
insert and Update involving those tables.
Seeing no data exists under section "Memory Resize Ops" and "Memory Resize
Operations Summary" for both issue and non issue period. So I hope it means
no memory movement happens in/out of the buffer cache.
CPU and other resources noted under the "Streams section" of the AWR
report are under ~10minutes of DB time. So I hope that may not be a
contributing factor here.
With regards to total memory being ~500GB+ and 60% of this is free on this
host. Currently for this database we have ~24GB allocated to SGA and ~12GB
for PGA_Aggregate_target. So by what amount of additional memory should we
bump the SGA_MAX_SIZE here?
I am now seeing there are ~19 tables which are assigned to the KEEP
pool(which is ~8GB in size) , but the sum of the size of all those ~19
tables are coming around ~13GB. And the odd thing is one of those tables is
exceeding ~12Gb itself in size. So i need to check if it's really serving
some purpose or we can remove the KEEP pool and merge it to the default
buffer cache only.
Regards
Pap
On Mon, Jan 18, 2021 at 2:15 AM Jonathan Lewis <jlewisoracle@xxxxxxxxx>
wrote:
I made two mistakes reading the details about parsing -
a) I forgot the "parse time" in the profile was in hundredths of
seconds
b) I took the library cache mutex X at 2,200 seconds to be reasonably
consistent with the 3600 figure for parse time, but since that was actually
36 seconds, and since it wasn't CURSOR mutex X I was putting 2 and 2
together to get 5.
So the parsing thing is probably mostly irrelevant.
Don't be fooled by the fact I typed MB instead of GB - you have 512GB of
memory and a huge fraction of it is "free". Why isn't more given to the
database.
We're also beginning to see some detail of what you're up to: your title
is High Free Buffer Waits, but it's taken about 10 posts before you mention
that you've got a KEEP cache and a DEFAULT cache, and that there are
several GB of SGA which won't be available to the shared_pool or buffer
cache because they are allocated to the Streams Pool. (Find out what
you're application is supposed to be doing in that 15 minutes and ask
yourself whether that might have some extra impact because of Streams;
check the Streams section of the AWR report to see if there is any Streams
activity going on at the time, or at all).
Since you've got a KEEP cache and a DEFAULT cache, can you identify which
cache is subject to the free buffer waits. (There's a very old article
about the oddities that used to appear with multiple caches here:
https://jonathanlewis.wordpress.com/2006/11/21/free-buffer-waits/ that
might help, but there's a "Buffer Pool Statistics" section of the AWR
report that gives you a simple summary.) Knowing which objects are
associated with the KEEP pool may also be helpful. And the "Segments by
..." sections of the AWR will give you some information about which objects
are subject to most changes in the period. Combine that with "SQL ordered
by ..." (Executions is the obvious starting point) to get some idea of
what's happening in the interval if you have no other way of finding out
what the application is supposed to be doing. Bear in mind that "%
captured" is reported for several of these reports, so it's important to
check if you're looking at a meaningfull percentage or whether some figure
looks harmless because it's only a small percentage of the real total.
Another thing to check (which MAY be related to the library cache mutex
X) is the section on memory resizing - do you have much activity of
granules of memory being moved between the library cache the streams pool,
and the buffer cache.
Regards
Jonathan Lewis
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On Sun, 17 Jan 2021 at 19:31, Pap <oracle.developer35@xxxxxxxxx> wrote:
Thanks much, Jonathan.
With respect to parsing , i have attached the "time model
statistics' section (for issue period and normal period in two tabs) and it
shows , the parse time is <1% of the over all DB time and even the top sql
by parsing , the top one which is shown as ~2.7million execution does use
bind variable, its something as this. (SELECT MAX (col1) FROM TAB1 WHERE
ID1 = :B2 AND date1 = :B1;). So then, can we safely say parsing(hard
parsing) to not be one of the causes?
To your point "are you "*wasting" a lot of memory (and CPU) to manage a
file-system. If not, how hard are the other applications / databases on
this machine working during this period*."
Not heard of any issue from other databases exactly during the same
point in time but yes they have very less activity happen on them and i do
see a small spike in concurrency during that exact time on them too. But ,
if I see the host memory utilization pattern from the last 7 days it's
always lying below ~40%. And the CPU utilization is max ~75% but it's not
showing a spike during that ~15minutes window. I am checking this on the
OEM Host utilization section , so not sure if it's hiding anything here.
And as I mentioned, the spike in AAS in the top activity section of this
database is clearly visible daily for that ~15minutes window in the top
activity section of OEM which touched to ~80(~64 being the limit).
Also Jonathan to your point *"and your SGA size is only 24MB so you may
be trying to recycle your buffer cache incredibly quickly.", *the SGA
is actually set ~24GB , however if i see some of the major components hard
coded and are in GB's , they are as 1) Keep buffer cache set as ~8GB ,
2)Default buffer cache - 6GB, 3)Shared pool - 4Gb and 4)streams pool- 5GB.
So does it mean that we really need to bump up the size of SGA so that the
default buffer cache can get more memory which may help us to reduce some
contention because the default buffer cache recycle frequency can be
minimized ?
Regards
Pap
On Sun, Jan 17, 2021 at 9:31 PM Jonathan Lewis <jlewisoracle@xxxxxxxxx>
wrote:
1) No.
2) No
Going back to first principles - typical Unix systems time-slice CPUs
1/100 of a second at a time and OLTP systems do tiny amounts of work that
are likely to take far less than 1/100 of a second if they're written
efficiently. So a well-written OLTP system can probably support a few tens
of user sessions per (real) CPUs because most of the session will spend
most of their time not doing anything.
If, however, you have 64 CPUs and 64 sessions which are trying to model
protein folding images from a chemical formula then you've allocated too
many sessions because every single one of them will want 100% of a CPU 100%
of the time and you've got no CPU left for anything else. More
realistically, if you have a couple of sessions which are trying to do
something resource intensive like running a query over the last 2 years of
sales data to produce a one page summary of customer trends then they are
probably taking out one CPU per session (or N CPUs each if you run the
queries parallel N). So rule 1 is "know your application"
Now take a look at the two bits of AWR you first posted.
1) The average single block read time in BOTH time periods is around
1ms - which means its coming from cache or flash; so it's likely to be more
CPU than I/O time.
2) The number of db block changes in the heavy period is about 60,000
per SECOND, compared to about 11,000. (Halve these figures since roughly
half will be for undo records going to pinned blocks) - so in the busy time
period you are potentially make 30,000 blocks dirty per SECOND and will
need to write them out, which is about 240MB per second, and your SGA size
is only 24MB so you may be trying to recycle your buffer cache incredibly
quickly.
3) As a side effect of the db block changes you are generating 12.7MB
of redo per second compared to 2.8MB per second. In a production system not
only does that have to be written to at least 2 files, it has to be re-read
and copied to an archived log destination.
Combining 2 and 3 it's not surprising that the rate of data change
means you have a huge amount of conflict in (a) writing data (b) finding
free buffers to read more data and (c) waitinig for other users to read
the data you want to read.
It's not easy to determine cause and effect after this - but the rate
at which you work in that 15 minutes is simply overloading the system. So
the first thing I'd do is try to find out what that 64 CPUs / 64 Cores
really means. HP/UX is one of the less well-known operating systems, so
any comment you hear about "unix systems" might be very misleading - but to
me it looks as if you may not really have 64 CPUs. Then I'd ask about the
I/O configuration - I know HP/UX is a bit unusual here, but what are the
setting to avoid double caching - you have 512GB of RAM and assign 24GB to
this SGA - are you "wasting" a lot of memory (and CPU) to managing a
file-system. If not, how hard are the other applications / databases on
this machine working during this period.
The other thing that stands out as a separate anomaly is the parsing -
which ends up with lots of "lilbrary cache - mutex X". The counts are high
in both cases and worth looking into, but very expensive in the worst
case. How many of your "parse count" turn into "parse count (hard)", and
what does the time model show as the distribution of parse time. At a guess
I'd say that in this interval you kick of several processes (maybe around
10) that are all trying to optimize the same statements at the same time -
which leads to the "mutex X" (the rough idea is that 9 out of 10 would be
waiting while the 10th does the optimising) - anifd maybe there are a lot
of statments which need a lot of parsing in an environment that is already
generaing a lot of CPU conflict. Note that your EXECUTE count jumps from
6,400 per second to 13,200 per second - you mentioned single-row
processing, the numbers suggest that that might be the case, but also that
a lot of them use literal values, and several sessions may be trying to
parse and execute the same statement at the same time.
Summary:
Check your system and how its configured.
Understand what the application is doing in this 15 minutes
Then worry about getting the same job done with less work and
(particularly) less conflict.
Regards
Jonathan Lewis
On Sun, 17 Jan 2021 at 11:18, Pap <oracle.developer35@xxxxxxxxx> wrote:
Basically I have two doubts here,
As it seems for 15minutes period we are pushing the System beyond its
capacity. So considering we have 64 core CPU here , should we only allow
max 64 active session at any given point from application? And then
considering we have 4 app server we need to set max session limit to 16
from each app server?
Another doubt I had was if the dB writer response time of 100ms
during that issue period is the cause or the result of slowness? If it's
the cause can asynch IO setup for data file help improve situation here?
Or
by just making row by row operations converted to batch will be enough ?
Regards
Pap
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