[mapguide-internals] RE: PATCH: Raster stability fixes, ticket#462

Haris Kurtagic haris at sl-king.com
Sun Mar 29 04:54:03 EDT 2009 

Hi Traian,
by just adding ref count to FDO GDA provider you did went into lock,
with single connection to provider. 
That is where connection manager is wrong, it was constantly waiting for
connection (it is not problem of guard in stylization). 
By enabling connection pooling it is still single thread because
provider itself is single threaded.
With only ref count in FDO GDAL , MG will not work properly.

I think we need to slow down a bit and find a solution for all issues we
have there. It is a connected problem and we need to find solution for
few related problems.

- Connection manager not using connection ref count for deciding
used/unused connections
- Really allow executing of single threaded providers
- I have feeling that because some problems previously were tried to
solve too quickly we ended up with gdal blocked all the way, perhaps
that is not needed.

Haris

-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org
[mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Traian
Stanev
Sent: Sunday, March 29, 2009 7:22 AM
To: MapGuide Internals Mail List
Cc: FDO Internals Mail List
Subject: RE: [mapguide-internals] RE: PATCH: Raster stability fixes,
ticket#462


For what it's worth, patching the provider to addref the parent
connection from the feature reader fixes the problem. This is on top of
Friday's trunk MapGuide and FDO code, without any other extra fixes. I
do think it is a bug not to addref the connection from the reader, when
it in fact depends on it to be alive, in order to get stuff from it at a
later time.

I am pretty sure the addref fixes the problem, because the test I am
using is fairly brutal and produces the crash/deadlock in less than 5
seconds every time I run it with unpatched GDAL provider. With the
change, I ran it multiple times to completion.

I am unhappy with the fix though, because it leaves CPU usage by
mgserver at ~5%, even though I am doing something like 16 simultaneous
requests... So then I decided to remove the global mutex from inside the
provider -- that didn't make a difference in CPU usage, and the test
still passed without problems. So that mutex can go. Then I messed with
the connection pooling settings in serverconfig.ini -- those didn't make
any difference. I tried them all for the GDAL provider and the test
still passed with 5% CPU usage. I don't think the process is blocked for
I/O because the TIFF I am using is not huge and I also generated an
overlay pyramid (which I highly recommend to anyone by the way, it makes
a huge performance difference for TIFFs).




Traian


> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org [mailto:mapguide- 
> internals-bounces at lists.osgeo.org] On Behalf Of Haris Kurtagic
> Sent: Saturday, March 28, 2009 4:43 AM
> To: MapGuide Internals Mail List
> Subject: RE: [mapguide-internals] RE: PATCH: Raster stability fixes, 
> ticket #462
> 
> Regarding MG raster problems ( or any other provider ) the main reason

> is connection manager.
> If you would set connection pool size 1 , for sdf provider MG will not

> work properly.
> 
> The patch Jason submitted, is workaround for issue. It will not cause 
> GDAL to work slower, because provider itself is basically 
> single-threaded now.
> That was done trough work we were doing years or so ago. I was looking

> at provider back then and realized that making gdal calls guarded will

> make it more stable. Frank then submitted patch with guard on every 
> single gdal library call.
> 
> I will try to explain what is happing now:
> 1. Map with more than one gdal layer
> 2. There are two calls in server code execute query and stylize 3. One

> thread will open connection in execute query and went to stylize 
> procedure 4. Another thread will come into execute query and because 
> it could be another layer it will try to open new connection 5. 
> Connection manager will check pool size ( 1 in gdal case ) and because

> provider didn't inc. reference count to connection (not  way to solve,

> but anyhow provider needs to increment ) it will delete previous 
> connection and create new one 6. Access violation in first thread 
> while stylization because of deleted connection
> 
> 
> In shorter version things to change:
> 1. FDO GDAL provider is not increasing reference count on FDO 
> connection when used in other classes like georasterband , which is 
> later used in query (wrong) 2. connection manager is managing 
> connection pool by using fdo connection reference count ( wrong ) 3. 
> it looks like working for other providers, because connection pool 
> size is 200 and other providers are incrementing connection pointer in

> select command for example, but if there would be provider with select

> command with no reference count to connection, it will crash too 4. 
> Server code is making assumption that pool size for single threaded is

> 1 (not correct), we need to make code change to allow for making calls

> to provider single threaded
> 
> Haris
> 
> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org
> [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Bruce

> Dechant
> Sent: Friday, March 27, 2009 4:55 PM
> To: MapGuide Internals Mail List
> Subject: [mapguide-internals] RE: PATCH: Raster stability fixes, 
> ticket
> #462
> 
> I totally agree - the locking needs to be in the provider.
> 
> Thanks,
> Bruce
> 
> 
> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org
> [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of 
> Traian Stanev
> Sent: Friday, March 27, 2009 9:40 AM
> To: MapGuide Internals Mail List
> Subject: [mapguide-internals] RE: PATCH: Raster stability fixes, 
> ticket
> #462
> 
> 
> It does look like a sledgehammer approach -- what if some day there is

> a raster provider that works ok and doesn't need the locks? If 
> anything, the lock should be in the FDO provider.
> 
> Traian
> 
> 
> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org
> [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Walt 
> Welton-Lair
> Sent: Friday, March 27, 2009 11:11 AM
> To: MapGuide Internals Mail List
> Subject: [mapguide-internals] RE: PATCH: Raster stability fixes, 
> ticket
> #462
> 
> The memory patch (mapguide_raster_unalloc.5.patch) looks good, and 
> I'll submit that.
> 
> The stability patch (mapguide_raster_stability.patch) looks 
> reasonable, but I'd like another 1 or 2 people to also look at it.  
> It's significant since it limits MapGuide to processing one raster 
> stylization request at a time, but I guess that's better than having a

> dead lock.  Anyone know where the thread-unsafe code actually is?  Is 
> it in the provider itself?
> Can the mutex be moved closer to the actual thread-unsafe code?
> 
> Walt
> 
> -----Original Message-----
> From: mapguide-internals-bounces at lists.osgeo.org
> [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Jason

> Birch
> Sent: Friday, March 27, 2009 3:10 AM
> To: mapguide-internals at lists.osgeo.org
> Subject: [mapguide-internals] PATCH: Raster stability fixes, ticket
> #462
> 
> Hi,
> 
> Haris has been working on the raster stability problem against 
> MapGuide 2.0.2.  I was having the same problems, and was able to 
> duplicate on 2.1.  I have worked with Haris to create patches against 
> trunk that address a couple of the problems and increase stability
immensely.
> 
> Could someone please review the patches attached to:
> 
> http://trac.osgeo.org/mapguide/ticket/462
> 
> and commit them to trunk if appropriate?
> 
> My feeling is that the connection management stuff may need some 
> additional work, since limiting the connection pool to 1 for 
> single-threaded providers doesn't appear to have the desired effect.
> However, getting both of these patches into 2.1 would at least improve

> stability for Raster users.
> 
> Jason_______________________________________________
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