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RE: Threads waiting on buffers

Posted By: Darrell Murphy
Date: Tuesday, 4 November 2003, at 1:33 p.m.

In Response To: Re: Threads waiting on buffers (Danny Wright)

Those of us running SAP have between 10,000 and 12,000 (yeah, the comma and
zeroes are correct) tables defined in the database. Hard to know how many
of those tables actually get used in any specific system. Using an 80/20
thing, we probably have at least 2,000 active tables but that's just a
guess. Running onstat -g dic the "number of dictionary entries" is usually
around 4,000 on my system. DD_HASSIZE is 613 here and DD_HASHMAX is 30. I
haven't been experiencing any of thrashing (that I know of). After this
issue though, I'm going to check that aspect more often. There are times
when things just seem to arbitrarily run slower. Or for some reason, a
checkpoint will take 10 minutes when they normally run 3 - 10 seconds. I'll
be including onstat -g dic as part of the checks I run when things seem
sluggish.

Darrell Murphy
Burton Snowboards

-----Original Message-----
From: Danny Wright [mailto:dwright@sherwoodfoods.com]
Sent: Tuesday, November 04, 2003 11:41 AM
To: ids@iiug.org
Subject: Re: Threads waiting on buffers [2126]

I haven't been keeping up - is 200 tables really all that much?

Has anyone experienced performance problems or anything else from having a
large number of tables? In our case, we're talking about 879 tables.

----- Original Message -----
From: "John Bejarano " <jbejarano@sbcglobal.net>
To: <ids@iiug.org>
Sent: Monday, November 03, 2003 2:43 PM
Subject: Re: Threads waiting on buffers [2122]

> So, yes, we do have over 200 tables. Including system
> catalog tables, there are 306 in this one database.
> And looking at onstat -g dic (for the first time), I
> see that 10 of the default 31 hash buckets are full
> with 10 entries each. Many others are 8 or 9. Art, I
> think you're showing your genius again. Thanks very
> much for the suggestion.
>
> Having brought this up with technical support, they
> recommend bringing DD_HASHSIZE up to 53 and DD_HASHMAX
> up to 20. Those values seem reasonable to me, I think
> we're going to go with them. (Shame this isn't
> documented anywhere.) While tech support won't say
> for sure that this could cause the issue we're seeing,
> they seem to agree it might. Has anyone else had
> their data dictionary thrash before? What effects did
> you see?
>
> Madison, my LRU MAX and MIN are set to 2 and 1.
>
> Thanks,
>
> --John Bejarano
> DBA, Shutterfly, Inc.
>
>
> --- "ART KAGEL, BLOOMBERG/ 65E 55TH"
> <KAGEL@bloomberg.net> wrote:
> > Do you have a large number of tables over all
> > databases on this instance (over
> > 200 or so)? Does onstat -g dic show the size column
> > value on most lines that
> > have an entry in the list# column are close to the
> > Maximum list size value shown
> > at the top? Do you have a relatively large number
> > of tables with similar names
> > that might all be hashing to the same one or two
> > data dictionary buckets?
> >
> > If so it could be that the Data Dictionary cache is
> > thrashing causing the
> > dictionary entries in the cache for some active
> > tables to be read from disk
> > frequently. At peak this causes the buffer access
> > storm on systables entries to
> > reload the dictionary cache. Try increasing the
> > data dictionary cache size.
> > The ONCONFIG parameters are:
> >
> > DD_HASHSIZE - This must be prime. It is the number
> > of hash table buckets unless
> > you have a large number of tables with similar
> > names increase this parameter
> > before you play with DD_HASHMAX. (Default: 31)
> > DD_HASHMAX - The number of slots in each hash
> > bucket, if more than this number
> > of tablenames hash to the same value because their
> > names are very similar then
> > increasing this one will remove the thrashing.
> > (Default: 10)
> >
> > Art S. Kagel
> >
>

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<TITLE>RE: Threads waiting on buffers [2126] </TITLE>
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Those of us running SAP have between 10,000 and =
12,000 (yeah, the comma and zeroes are correct) tables defined in the =
database.  Hard to know how many of those tables actually get used =
in any specific system.  Using an 80/20 thing, we probably have at =
least 2,000 active tables but that's just a guess.  Running onstat =
-g dic the "number of dictionary entries" is usually around =
4,000 on my system.  DD_HASSIZE is 613 here and DD_HASHMAX is =
30.  I haven't been experiencing any of thrashing (that I know =
of).   After this issue though, I'm going to check that =
aspect more often.  There are times when things just seem to =
arbitrarily run slower.  Or for some reason, a checkpoint will =
take 10 minutes when they normally run 3 - 10 seconds.  I'll be =
including onstat -g dic as part of the checks I run when things seem =
sluggish.

Darrell Murphy
 Burton Snowboards


 -----Original Message-----
 From:   Danny Wright [<A =
HREF=3D"mailto:dwright@sherwoodfoods.com">mailto:dwright@sherwoodfoods.c=
om</A>] 
 Sent:   Tuesday, November 04, 2003 11:41 =
AM
 To:     ids@iiug.org
 Subject:        =
Re: Threads waiting on buffers    [2126] 


I haven't been keeping up - is 200 tables really all =
that much?


Has anyone experienced performance problems or =
anything else from having a
 large number of tables?  In our case, we're =
talking about 879 tables.


----- Original Message ----- 
 From: "John Bejarano " =
<jbejarano@sbcglobal.net>
 To: <ids@iiug.org>
 Sent: Monday, November 03, 2003 2:43 PM
 Subject: Re: Threads waiting on buffers =
[2122]

 

> So, yes, we do have over 200 tables.  =
Including system
 > catalog tables, there are 306 in this one =
database.
 > And looking at onstat -g dic (for the first =
time), I
 > see that 10 of the default 31 hash buckets are =
full
 > with 10 entries each.  Many others are 8 =
or 9.  Art, I
 > think you're showing your genius again.  =
Thanks very
 > much for the suggestion.
 >
 > Having brought this up with technical support, =
they
 > recommend bringing DD_HASHSIZE up to 53 and =
DD_HASHMAX
 > up to 20.  Those values seem reasonable to =
me, I think
 > we're going to go with them.  (Shame this =
isn't
 > documented anywhere.)  While tech support =
won't say
 > for sure that this could cause the issue we're =
seeing,
 > they seem to agree it might.  Has anyone =
else had
 > their data dictionary thrash before?  What =
effects did
 > you see?
 >
 > Madison, my LRU MAX and MIN are set to 2 and 1.<=
/FONT>
 >
 > Thanks,
 >
 > --John Bejarano
 > DBA, Shutterfly, Inc.
 >
 >
 > --- "ART KAGEL, BLOOMBERG/ 65E =
55TH"
 > <KAGEL@bloomberg.net> wrote:
 > > Do you have a large number of tables over =
all
 > > databases on this instance (over
 > > 200 or so)?  Does onstat -g dic show =
the size column
 > > value on most lines that
 > > have an entry in the list# column are =
close to the
 > > Maximum list size value shown
 > >  at the top?  Do you have a =
relatively large number
 > > of tables with similar names
 > >  that might all be hashing to the =
same one or two
 > > data dictionary buckets?
 > >
 > > If so it could be that the Data Dictionary =
cache is
 > > thrashing causing the
 > > dictionary entries in the cache for some =
active
 > > tables to be read from disk
 > > frequently.  At peak this causes the =
buffer access
 > > storm on systables entries to
 > >  reload the dictionary cache.  =
Try increasing the
 > > data dictionary cache size.
 > > The ONCONFIG parameters are:
 > >
 > > DD_HASHSIZE - This must be prime.  It =
is the number
 > > of hash table buckets unless
 > >  you have a large number of tables =
with similar
 > > names increase this parameter
 > > before you play with DD_HASHMAX. (Default: =
31)
 > > DD_HASHMAX - The number of slots in each =
hash
 > > bucket, if more than this number
 > > of tablenames hash to the same value =
because their
 > > names are very similar then
 > > increasing this one will remove the =
thrashing.
 > > (Default: 10)
 > >
 > > Art S. Kagel
 > >
 >


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Messages In This Thread

Threads waiting on buffers
John Bejarano -- Monday, 3 November 2003, at 11:15 a.m.
- Re: Threads waiting on buffers
  Madison Pruet -- Monday, 3 November 2003, at 3:05 p.m.
- Re: Threads waiting on buffers
  John Bejarano -- Monday, 3 November 2003, at 4:43 p.m.
  - Re: Threads waiting on buffers
    Danny Wright -- Tuesday, 4 November 2003, at 11:41 a.m.
    - RE: Threads waiting on buffers
      Darrell Murphy -- Tuesday, 4 November 2003, at 1:33 p.m.
    - RE: Threads waiting on buffers
      Javier Zayas -- Tuesday, 4 November 2003, at 1:52 p.m.
    - Re: Threads waiting on buffers
      ART KAGEL, BLOOMBERG/ 65E 55TH -- Tuesday, 4 November 2003, at 2:11 p.m.
      - RE: Threads waiting on buffers
        Meza, Silvana -- Wednesday, 5 November 2003, at 3:47 p.m.
    - Re: Threads waiting on buffers
      Martin Fuerderer -- Wednesday, 5 November 2003, at 3:50 a.m.
    - Re: Threads waiting on buffers
      Obnoxio The Clown -- Friday, 7 November 2003, at 4:06 a.m.

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