On Tue, 2008-06-10 at 10:40 +0200, Frederik Ramm wrote:
> Hi,
>
> > Um... no. At least not if it does the sensible thing and maps it
> > for shared read.
>
> I don't think it is even possible to mmap somthing for "private read".
> The boost lib only ever sets the SHARED flag when it wants write access:
>
> void* data = ::mmap( hint, pimpl_->size_,
> readonly ? PROT_READ : (PROT_READ |PROT_WRITE),
> readonly ? MAP_PRIVATE : MAP_SHARED,
> pimpl_->handle_, p.offset );
>
> I guess that a "shared" mode is implicitly used by mmap if you specify
> read access only; anything else would not make sense (and would mean
> that the boost guys got it wrong, which is not impossible but rare).
Those lines above imply that readonly mappings are flagged MAP_PRIVATE
which looks a little odd. The the strace output below seems to confirm
these these flags:
23807 open("/home/jburgess/osm/data/world_boundaries/processed_p.shp",
O_RDONLY) = 18
** 23807 mmap(NULL, 327324396, PROT_READ, MAP_PRIVATE, 18, 0) = 0x7f46842d9000
23807 open("/home/jburgess/osm/data/world_boundaries/processed_p.dbf",
O_RDONLY) = 19
23807 mmap(NULL, 9758339, PROT_READ, MAP_PRIVATE, 19, 0) = 0x7f469a477000
23807 open("/home/jburgess/osm/data/world_boundaries/processed_p.index",
O_RDONLY <unfinished ...>
23808 open("/home/jburgess/osm/data/world_boundaries/processed_p.shp", O_RDONLY
<unfinished ...>
23807 <... open resumed> ) = 20
23808 <... open resumed> ) = 21
23807 mmap(NULL, 2268216, PROT_READ, MAP_PRIVATE, 20, 0) = 0x7f46840af000
** 23808 mmap(NULL, 327324396, PROT_READ, MAP_PRIVATE, 21, 0) = 0x7f4670885000
The two lines ** are memory maps of the same 300MB shapefiles though the
fd's are different. This is being done concurrently by 2 threads and the
resulting address mappings returned are different.
Jon
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