CR is the most popular technique in HPC and can be implemented at
application-level implementation, user-level implementation and system level
implementation. Application level checkpoint services interact directly with
the application to capture the state of the program 75. Application-level
checkpointing accommodates heterogeneous systems, but lacks transparency, which
is usually available with a kernel-level or a user-level approach. The major
challenge in this approach is that it requires the programmer to have a good
understanding of the applications to be checkpointed. The Cornell
Checkpoint(pre) Compiler (C3) 66 is an excellent implementation of application-level
checkpointing.  With user-level
implementations, a user-level library is used to do the checkpointing and the
application programs are linked to the library. Some typical library
implementations are Esky 28, Condo 72, and libckpt 56. This approach is
usually not transparent to users because applications are modified, recompiled
and relinked to the checkpoint library before the checkpoint facility is used.

Checkpoint/restart may be implemented at the system level, either
in the OS kernel or in hardware. Examples of system-level implementations
include CRAK 79, Zap 51, and BLCR 5, 6, 7, 8 library implements kernel level checkpoint and
is used in some production clusters.. One drawback, however, is that a kernel
level implementation is not portable to other platforms 66.

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Various optimization techniques have been studied to improve both
forms of overhead, a short description of which follows.  Optimizations of coordinated CR such as
incremental 5, 6   incremental
checkpoint with the Libckpt library 9, and speculative checkpoint 10, which
is an attemp to exploit temporal reduction in system level checkpointing.

, non-blocking 7   Attempts
have been made to create nonblocking or semi-blocking checkpointing, which
allows the system to continue to execute while checkpoints are saved to
permanent storage 12 13. , 

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