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SEC1 doesn't support S/G in descriptors so for hash operations, the CPU has to build a buffer containing the buffered block and the incoming data. This generates a lot of memory copies which represents more than 50% of CPU time of a md5sum operation as shown below with a 'perf record'. |--86.24%-- kcapi_md_digest | | | |--86.18%-- _kcapi_common_vmsplice_chunk_fd | | | | | |--83.68%-- splice | | | | | | | |--83.59%-- ret_from_syscall | | | | | | | | | |--83.52%-- sys_splice | | | | | | | | | | | |--83.49%-- splice_from_pipe | | | | | | | | | | | | | |--83.04%-- __splice_from_pipe | | | | | | | | | | | | | | | |--80.67%-- pipe_to_sendpage | | | | | | | | | | | | | | | | | |--78.25%-- hash_sendpage | | | | | | | | | | | | | | | | | | | |--60.08%-- ahash_process_req | | | | | | | | | | | | | | | | | | | | | |--56.36%-- sg_copy_buffer | | | | | | | | | | | | | | | | | | | | | | | |--55.29%-- memcpy | | | | | | | | | | | | However, unlike SEC2+, SEC1 offers the possibility to chain descriptors. It is therefore possible to build a first descriptor pointing to the buffered data and a second descriptor pointing to the incoming data, hence avoiding the memory copy to a single buffer. With this patch, the time necessary for a md5sum on a 90Mbytes file is approximately 3 seconds. Without the patch it takes 6 seconds. Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.