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@@ -46,6 +46,12 @@ disk, e.g. a file name in the \c stdio or \c unistd cases, or a database key in
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data handle. StarPU will then use this file as external source of data, and
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automatically read and write data as appropriate.
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+In any case, the user also needs to set STARPU_LIMIT_CPU_MEM to the amount of
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+data that StarPU will be allowed to afford. By default StarPU will use the
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+machine memory size, but part of it is taken by the kernel, the system,
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+daemons, and the application's own allocated data, whose size can not be
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+predicted. That is why the user needs to specify what StarPU can afford.
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+
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\section UseANewDiskMemory Use a new disk memory
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To use a disk memory node, you have to register it with this function:
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@@ -68,10 +74,19 @@ export STARPU_DISK_SWAP_BACKEND=unistd
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export STARPU_DISK_SWAP_SIZE=200
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\endverbatim
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-The backend can be set to \c stdio (some caching is done by \c libc), \c unistd (only
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+The backend can be set to \c stdio (some caching is done by \c libc and the kernel), \c unistd (only
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caching in the kernel), \c unistd_o_direct (no caching), \c leveldb, or \c hdf5.
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-When that register call is made, StarPU will benchmark the disk. This can
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+It is important to understand that when the backend is not set to \c
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+unistd_o_direct, some caching will occur at the kernel level (the page cache),
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+which will also consume memory... STARPU_LIMIT_CPU_MEM might need to be set
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+to less that half of the machine memory just to leave room for the kernel's
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+page cache, otherwise the kernel will struggle to get memory. Using \c
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+unistd_o_direct avoids this caching, thus allowing to set STARPU_LIMIT_CPU_MEM
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+to the machine memory size (minus some memory for normal kernel operations,
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+system daemons, and application data).
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+
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+When the register call is made, StarPU will benchmark the disk. This can
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take some time.
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<strong>Warning: the size thus has to be at least \ref STARPU_DISK_SIZE_MIN bytes ! </strong>
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