HugePages – Overhead of Not Using

I’ve read a fair bit about HugePages and their importance especially for Oracle databases with large SGAs and a large number of dedicated connections.

Admittedly, some of what I read is over my head – I’m not a production DBA and quite often I don’t even have access to the box unfortunately – but I know who the experts are and I trust them (doesn’t mean that an expert’s opinion shouldn’t be tested of course):

Now, the title in Kevin’s posts asks ( rhetorically) “Configuring Linux Hugepages for Oracle Database Is Just Too Difficult! Isn’t It?”

And I think a lot of the time, it is. Not because of the topic but because of the structure of modern IT departments.

When I tried to tackle the subject before at a client, it was awkward – it wasn’t a subject I could talk about with any great authority although I could refer them to the expert material above and … well, to cut a short story shorter and keep me out of trouble… we didn’t. I was overruled.  No-one else at the client was using HugePages on 11g. So no HugePages and no chance to test it.

Well, in the end, we didn’t go with AMM, because for our applications, according to our tests, we were better suited to ASMM (damn these acronyms).

Statements like this from Kevin’s articles above are easy to understand:

Reasons for Using Hugepages

  1. Use hugepages if OLTP or ERP. Full stop.
  2. Use hugepages if DW/BI with large numbers of dedicated connections or a large SGA. Full stop.
  3. Use hugepages if you don’t like the amount of memory page tables are costing you (/proc/meminfo). Full stop.

and

“Large number of dedicated connections” and “large SGA” in the context of hugepages can only be quantified by the amount of memory wasted in page tables and whether the administrator is satisfied with that cost.”

Those are pretty clear cut.

So, if I understand everything correctly, I should not like this, right?:

$ grep "PageTables" /proc/meminfo
PageTables:   41622228 kB
$

That’s 40 gig of overhead for an SGA of 60 gig and a whole bunch of connections.
So, don’t need HugePages, eh?
Bit late now.

Why did that report run slow? ASH says log file sequential read

“Dump Online Redo for Buffers in Pin History”

I’ve just been investigating why a certain report ran slow last night.
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Regexp hungry for CPU? Real time sql monitoring shows just how

Not exactly a snappy blog post title…

First up, an execution plan showing the old problem of how the costs of a scalar subquery are not properly accounted for, even in the latest 11gR2 releases.
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Poll: SQL Plan Management

I’m interested in what SQL Plan Management features people are actively using.
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ORA-00600 [kkqtSetOp.1] – Join Factorization

Just a quick note about an ORA-00600 that recently occurred following an upgrade to 11.2.0.3.

I’ve not been able to distill an isolated test case from the specific production code generating this error so I’ll skip the application-specific example.

The error message reported was:
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Timeslot SQL

A reminder about sql logic required to deal with express datetimes to the nearest x mins – for example, this particular question come up on an internal forum last week where the poster wanted to aggregate data per quarter hour.

There are two approaches depending on the exact requirement.

The first option is to generate the timeslots using a data generator and then join to the source data.

SQL>  alter session set nls_date_format = 'DD-MON-YYYY Hh24:MI';

Session altered.

SQL> with times as
  2  (select trunc(sysdate) + ((rownum-1)*15/60/24) slot
  3   from   dual
  4   connect by rownum <= 10)  --<-- just to keep the output short
  5  select * from times;

SLOT
-----------------
30-APR-2012 00:00
30-APR-2012 00:15
30-APR-2012 00:30
30-APR-2012 00:45
30-APR-2012 01:00
30-APR-2012 01:15
30-APR-2012 01:30
30-APR-2012 01:45
30-APR-2012 02:00
30-APR-2012 02:15

10 rows selected.

The second options is to round/floor/ceil the relevant existing date/timestamp in the table to the nearest x minutes.

For this solution, there is a generic approach as follows, demonstrated using sysdate:

SQL> select sysdate,
  2         trunc(sysdate)
  3       + floor(to_char(sysdate,'sssss') / 900) / 96 floor_slot,
  4         trunc(sysdate)
  5       + round(to_char(sysdate,'sssss') / 900) / 96 round_slot,
  6         trunc(sysdate)
  7       + ceil(to_char(sysdate,'sssss') / 900) / 96 ceil_slot
  8  from   dual;

SYSDATE           FLOOR_SLOT        ROUND_SLOT        CEIL_SLOT
----------------- ----------------- ----------------- -----------------
30-APR-2012 09:58 30-APR-2012 09:45 30-APR-2012 10:00 30-APR-2012 10:00

SQL> 

Where the basic forumla is:

 trunc(sysdate)
+ floor(to_char(sysdate,'sssss') / x ) / y 
  1. The date – example uses sysdate but this would probably be a column – truncated to start of day.
  2. The usage of FLOOR / ROUND / CEIL depends on the requirement.
  3. to_char(sysdate,’sssss’) is the number of seconds since midnight
  4. x is the number of seconds in the timeslot length we’re interested in – for the example, 15 minutes * 60 seconds per minute = 900
  5. Because date arithmetic is done in days, y is the number of timeslots in a day – for this example 4 timeslots per hour * 24 hours per day = 96

So, if we want five minute timeslots, x is 5*60=300; y is 12*24=288:

SQL> select sysdate,
  2         trunc(sysdate)
  3       + floor(to_char(sysdate,'sssss') / 300) / 288 floor_stamp,
  4         trunc(sysdate)
  5       + round(to_char(sysdate,'sssss') / 300) / 288 round_stamp,
  6         trunc(sysdate)
  7       + ceil(to_char(sysdate,'sssss') / 300) / 288 ceil_stamp
  8  from   dual;

SYSDATE           FLOOR_STAMP       ROUND_STAMP       CEIL_STAMP
----------------- ----------------- ----------------- -----------------
30-APR-2012 10:13 30-APR-2012 10:10 30-APR-2012 10:15 30-APR-2012 10:15

SQL> 

Or, for half hour timeslots, x is 60*30=1800; y is 2*24=48:

SQL> select sysdate,
  2         trunc(sysdate)
  3       + floor(to_char(sysdate,'sssss') / 1800) / 48 floor_stamp,
  4         trunc(sysdate)
  5       + round(to_char(sysdate,'sssss') / 1800) / 48 round_stamp,
  6         trunc(sysdate)
  7       + ceil(to_char(sysdate,'sssss') / 1800) / 48 ceil_stamp
  8  from   dual;

SYSDATE           FLOOR_STAMP       ROUND_STAMP       CEIL_STAMP
----------------- ----------------- ----------------- -----------------
30-APR-2012 10:19 30-APR-2012 10:00 30-APR-2012 10:30 30-APR-2012 10:30

SQL> 

Deadlock trace file enhancements

Just browsing some deadlock trace files on 11gR2.

I normally look at sections:

  • Deadlock Graph
  • Information for the OTHER waiting session
  • Information for THIS session

and very rarely go beyond section PROCESS_STATE.

But, casually browsing, I came across these sections relating to the killed session:

  • Session Wait History
  • Sampled Session History of session

Nice additions. When did that happen?

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