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Some architectures have a set of zero pages (coloured zero pages) instead of only one zero page, in order to improve the cache performance. In those cases, the kernel samepage merger (KSM) would merge all the allocated pages that happen to be filled with zeroes to the same deduplicated page, thus losing all the advantages of coloured zero pages. This behaviour is noticeable when a process accesses large arrays of allocated pages containing zeroes. A test I conducted on s390 shows that there is a speed penalty when KSM merges such pages, compared to not merging them or using actual zero pages from the start without breaking the COW. This patch fixes this behaviour. When coloured zero pages are present, the checksum of a zero page is calculated during initialisation, and compared with the checksum of the current canditate during merging. In case of a match, the normal merging routine is used to merge the page with the correct coloured zero page, which ensures the candidate page is checked to be equal to the target zero page. A sysfs entry is also added to toggle this behaviour, since it can potentially introduce performance regressions, especially on architectures without coloured zero pages. The default value is disabled, for backwards compatibility. With this patch, the performance with KSM is the same as with non COW-broken actual zero pages, which is also the same as without KSM. [akpm@linux-foundation.org: make zero_checksum and ksm_use_zero_pages __read_mostly, per Andrea] [imbrenda@linux.vnet.ibm.com: documentation for coloured zero pages deduplication] Link: http://lkml.kernel.org/r/1484927522-1964-1-git-send-email-imbrenda@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1484850953-23941-1-git-send-email-imbrenda@linux.vnet.ibm.com Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Hugh Dickins <hughd@google.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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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.