[fdo-internals] RE: [mapguide-internals] RE: PATCH: Raster
stability fixes, ticket #462
Traian Stanev
traian.stanev at autodesk.com
Sun Mar 29 01:22:15 EDT 2009
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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