[vtk-developers] Memory-mapped files in VTK

[Berk Geveci]

Makes sense. Thanks for clarifying.


On Fri, May 16, 2014 at 2:28 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> Hi Berk,
>
> I was passing the data through a convolution filter immediately after
> mapping it, and what I was timing was how long it took for that
> operation to complete, for mmap vs. vtkImageReader2.
>
> However, the biggest advantage with mmap is when you are using
> the same file over and over again in different session (especially e.g.
> when you are debugging).  If you have a machine with loads of RAM,
> the whole file will remain in memory from one session to the next, so
> with mmap loading the file in subsequent sessions comes at zero cost.
> With a more typical reader, the file may still be cached in memory,
> but the reader is forced to copy the data from disk cache to a new array.
>
>   David
>
>
> On Fri, May 16, 2014 at 12:16 PM, Berk Geveci <berk.geveci at kitware.com>
> wrote:
> > I am not objecting to the concept but I have a question. Is it really
> > faster? Don't you have to take compute time into account if you have a
> > memory mapped file? That's when disk activity will actually occur. I am
> > wondering if this approach doesn't simply postpone the inevitable till
> > later. I understand this should be fast if processing only touches part
> of
> > the data but then it shouldn't be loaded in the first place.
> >
> > -berk
> >
> >
> >
> > On Fri, May 16, 2014 at 2:08 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
> >>
> >> Hi Burlen,
> >>
> >> Thanks for the links.  I guess that I'm looking for deeper integration
> >> (though less general) than a callback.  The idea is to have something
> >> like a vtkMemoryMap class to take care of all the platform-specific
> >> details of creating/deleting memory maps, and to have vtkAbstractArray
> >> be aware of this class and use it to delete mmaps when appropriate.
> >>
> >>   David
> >>
> >> On Fri, May 16, 2014 at 11:56 AM, Burlen Loring <
> burlen.loring at gmail.com>
> >> wrote:
> >> > meant to say "no special memory management issues"
> >> > and (in case you actually would look at the code) mmap use is here:
> >> >
> https://github.com/burlen/SC11-100GB-Demo/blob/master/SC11DemoReader.h
> >> >
> >> > On 05/16/2014 10:50 AM, Burlen Loring wrote:
> >> >
> >> > Hi David,
> >> >
> >> > I've used mmap with VTK in C++ for the SC11 100GB network demo.
> >> >
> >> > http://dl.acm.org/citation.cfm?id=2287004
> >> > https://github.com/burlen/SC11-100GB-Demo
> >> >
> >> > This use was very ad-hoc and we wanted the data mapped for the life of
> >> > the
> >> > app so there were special memory management issues.
> >> >
> >> > patch(http://review.source.kitware.com/#/c/14072/) would have handled
> >> > situations such as this through a callback invoked when the data array
> >> > is
> >> > finished with the pointer. However, that patch was rejected for being
> >> > overkill. Instead Berk suggested that I simply use the delete event
> >> > observer, which will always result in data being released but perhaps
> >> > not as
> >> > quickly as with the callback or the method that you proposed(eg
> nothing
> >> > happens if the array is resized). This approach has worked well for
> me.
> >> > I
> >> > wonder if using the delete event observer would work well in your
> case?
> >> >
> >> > Burlen
> >> >
> >> >
> >> > On 05/16/2014 10:21 AM, David Gobbi wrote:
> >> >
> >> > Hi All,
> >> >
> >> > I've used memory mapped files with VTK in the past, using the python
> >> > "mmap" object to create the map and then passing the resulting pointer
> >> > to VTK via the SetVoidArray() method.  For large files (several GB)
> I've
> >> > found this to be many times faster than e.g. vtkImageReader2.
> >> >
> >> > My question: has anyone used mmap with VTK in C++?  Specifically,
> >> > what I'm interested in doing is adding a new "delete method" to the
> >> > SetVoidPointer() method in vtkDataArrayTemplate.h: it already has
> >> > "delete methods" for "malloc" and "new", so the idea is to add another
> >> > delete method for "mmap" that will call munmap on the pointer.
> >> >
> >> >    David
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