/* StarPU --- Runtime system for heterogeneous multicore architectures.
 *
 * Copyright (C) 2013-2014,2016-2018                      CNRS
 * Copyright (C) 2013                                     Inria
 * Copyright (C) 2013-2014,2017-2018                      Université de Bordeaux
 * Copyright (C) 2013                                     Corentin Salingue
 *
 * StarPU is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or (at
 * your option) any later version.
 *
 * StarPU is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * See the GNU Lesser General Public License in COPYING.LGPL for more details.
 */

/*! \page OutOfCore Out Of Core

\section Introduction Introduction

When using StarPU, one may need to store more data than what the main memory
(RAM) can store. This part describes the method to add a new memory node on a
disk and to use it.

Similarly to what happens with GPUs (it's actually exactly the same code), when
available main memory becomes scarse, StarPU will evict unused data to the disk,
thus leaving room for new allocations. Whenever some evicted data is needed
again for a task, StarPU will automatically fetch it back from the disk.


The principle is that one first registers a disk location, seen by StarPU as a
<c>void*</c>, which can be for instance a Unix path for the stdio, unistd or
unistd_o_direct backends, or a leveldb database for the leveldb backend, an HDF5
file path for the HDF5 backend, etc. The disk backend opens this place with the
plug method.

StarPU can then start using it to allocate room and store data there with the
disk write method, without user intervention.

The user can also use starpu_disk_open() to explicitly open an object within the
disk, e.g. a file name in the stdio or unistd cases, or a database key in the
leveldb case, and then use <c>starpu_*_register</c> functions to turn it into a StarPU
data handle. StarPU will then use this file as external source of data, and
automatically read and write data as appropriate.

\section UseANewDiskMemory Use a new disk memory

To use a disk memory node, you have to register it with this function:

\code{.c}
int new_dd = starpu_disk_register(&starpu_disk_unistd_ops, (void *) "/tmp/", 1024*1024*200);
\endcode

Here, we use the unistd library to realize the read/write operations, i.e.
fread/fwrite. This structure must have a path where to store files, as well as
the maximum size the software can afford storing on the disk.

Don't forget to check if the result is correct!

This can also be achieved by just setting environment variables:

\verbatim
export STARPU_DISK_SWAP=/tmp
export STARPU_DISK_SWAP_BACKEND=unistd
export STARPU_DISK_SWAP_SIZE=200
\endverbatim

The backend can be set to stdio (some caching is done by libc), unistd (only
caching in the kernel), unistd_o_direct (no caching), leveldb, or hdf5.

When that register call is made, StarPU will benchmark the disk. This can
take some time.

<strong>Warning: the size thus has to be at least \ref STARPU_DISK_SIZE_MIN bytes ! </strong> 

StarPU will then automatically try to evict unused data to this new disk. One
can also use the standard StarPU memory node API to prefetch data etc., see the
\ref API_Standard_Memory_Library and the \ref API_Data_Interfaces .

The disk is unregistered during the starpu_shutdown().

\section OOCDataRegistration Data Registration

StarPU will only be able to achieve Out-Of-Core eviction if it controls memory
allocation. For instance, if the application does the following:

<code>
p = malloc(1024*1024*sizeof(float));
fill_with_data(p);
starpu_matrix_data_register(&h, STARPU_MAIN_RAM, (uintptr_t) p, 1024, 1024, 1024, sizeof(float));
</code>

StarPU will not be able to release the corresponding memory since it's the
application which allocated it, and StarPU can not know how, and thus how to
release it. One thus have to use the following instead:

<code>
starpu_matrix_data_register(&h, -1, NULL, 1024, 1024, 1024, sizeof(float));
starpu_task_insert(cl_fill_with_data, STARPU_W, h, 0);
</code>

Which makes StarPU automatically do the allocation when the task running
cl_fill_with_data gets executed. And then if its needs to, it will be able to
release it after having pushed the data to the disk.

\section OOCWontUse Using Wont Use

By default, StarPU uses a Least-Recently-Used (LRU) algorithm to determine
which data should be evicted to the disk. This algorithm can be hinted
by telling which data will no be used in the coming future thanks to
starpu_data_wont_use(), for instance:

<code>
starpu_task_insert(&cl_work, STARPU_RW, h, 0);
starpu_data_wont_use(h);
</code>

StarPU will mark the data as "inactive" and tend to evict to the disk that data
rather than others.

\section ExampleDiskCopy Examples: disk_copy

\snippet disk_copy.c To be included. You should update doxygen if you see this text.

\section ExampleDiskCompute Examples: disk_compute

\snippet disk_compute.c To be included. You should update doxygen if you see this text.

\section Performances

Scheduling heuristics for Out-of-core are still relatively experimental. The
tricky part is that you usually have to find a compromise between privileging
locality (which avoids back and forth with the disk) and privileging the
critical path, i.e. taking into account priorities to avoid lack of parallelism
at the end of the task graph.

It is notably better to avoid defining different priorities to tasks with low
priority, since that will make the scheduler want to schedule them by levels of
priority, at the depense of locality.

The scheduling algorithms worth trying are thus <code>dmdar</code> and
<code>lws</code>, which privilege data locality over priorities. There will be
work on this area in the coming future.

\section DiskFunctions Disk functions

There are various ways to operate a disk memory node, described by the structure
starpu_disk_ops. For instance, the variable #starpu_disk_unistd_ops
uses read/write functions.

All structures are in \ref API_Out_Of_Core.

*/