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IDS Forum

Re: VERY slow BTS index build

Posted By: Mark Ashworth
Date: Tuesday, 9 September 2014, at 11:17 a.m.

In Response To: Re: VERY slow BTS index build (MICHAEL HOFFMAN)

Hi Michael,

> It seems as if varchar
> works faster than char, but they take a few hours as well.

This is an interesting observation BTS will get trailing space
characters with CHAR that are stripped early in the index pipeline. Other
than that, from BTS, there should be no difference.

> Oh, and here's a CRAZY issue: immediately upon starting the build, the
engine
> dynamically adds *4* "new extension shared memory" segments! By the
> time I had
> stopped the build, *8 segnemts* had been added. This boosted the virtual

> memory to 2077696.
> Why would the engine need more VM? As I said, with the boost of
SHMVRTSIZE to
> 2GB, the entire database can fit there.

When BTS creates and index or inserts into an index, rows are inserted
into the index stages. The first stage is an small in-memory structure
called a ramdirectory. I typically run
with EXTSHMADD set to 8192 and it governs the number the size of the VX
class of memory segments. VX memory segments are associated with VPs like
the bts VP.
I have gone to 11.50.xC6 and trying the index build again. I do not see
a memory leak where the VX segments are growing:

IBM Informix Dynamic Server Version 11.50.FC6 -- On-Line -- Up
00:47:47 -- 72504 Kbytes

Segment Summary:
id key addr size ovhd class
blkused blkfree
19922948 52564801 44000000 7249920 517984 R 1768 2
19955717 52564802 446ea000 33439744 393376 V 7792
372
19988486 52564803 466ce000 8388608 99712 V 155

1893
20021255 52564804 46ece000 8388608 99712 VX 576

1472
20054024 52564805 476ce000 8388608 99712 VX 349

1699
20086793 52564806 47ece000 8388608 99712 VX 60

1988
Total: - - 74244096 - - 10700
7426

This has been steady for over an hour which also means
that 11.50.xC6 is much slower than 11.70.xC8W4 for BTS index builds.

Another post on this thread suggested using xact_ramdirectory="yes", this
will use considerably more memory for the index build because it would
result in the index being built in memory.

-- Mark.
Mark Ashworth
IBM Informix Extensibility Architect
Office phone: +1 (905) 413-5033
Alternate: +1 (905) 697-8094
Email: ashworth@ca.ibm.com
Check out my blog

ids-bounces@iiug.org wrote on 09/08/2014 06:44:39 PM:

> From: "MICHAEL HOFFMAN" <mrh@panix.com>
> To: ids@iiug.org,
> Date: 09/08/2014 06:45 PM
> Subject: Re: VERY slow BTS index build [33740]
> Sent by: ids-bounces@iiug.org
>
> Mark,
> That is the kind of response times I expect from Informix.
> I have no idea why these builds are taking so long. It seems as if
varchar
> works faster than char, but they take a few hours as well.
>
> This system is a fairly idle server, used for QA mostly. When the
> BTS build is
> running, 'top' shows the Informix oninits sitting above all else, and
the
> 'top' session next in line -- ie: nothing else is happening.
>
> I just ran a new test on a different column: char(25) with only *4*
> values for
> 980K rows. 850K are the same value. This should be a fairly simple index
to
> create ... after all, a regular index on the column completes in about 3

> minutes.
> After an hour-and-a-half, I stopped the test. No reason to wait any
longer,
> that's just too slow.
>
> I dropped the Buffers down to 200000 and LRUs at 8. Pushed the
SHMVRTSIZE to
> 2048000 -- the entire database should fit into that! SHMADD = 256000.
> EXTSHMADD = 4096.
>
> Looking at onstat -p, both Buffer Rates are above 99%. Good!
> Looking at onstat -p, both Buffer Rates are above 99%. Good!
> Looking at onstat -D shows some writes to bts_temp space (created
> the same way
> you did, with the '-t' option) and about 108000 reads from the databases

> dbspace.
> onstat -g opn shows several temp tables being opened and closed
constantly.
>
> Hmm... after an 1.5 hours, it only did 108K reads? BTS must be sorting
each
> row as it builds the index?
>
> Oh, and here's a CRAZY issue: immediately upon starting the build, the
engine
> dynamically adds *4* "new extension shared memory" segments! By the
> time I had
> stopped the build, *8 segnemts* had been added. This boosted the virtual

> memory to 2077696.
> Why would the engine need more VM? As I said, with the boost of
SHMVRTSIZE to
> 2GB, the entire database can fit there.
>
> Strange, huh? Art, you got any ideas?
>
> Thanks again! Really hoping to find an answer so I can convince the Big
Boys
> to sign off on using BTS.
>
> Michael
>
>
>
*******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>

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