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Top Forums UNIX for Advanced & Expert Users Memory fragmentation in a Linux settop box Post 302908822 by achenle on Thursday 10th of July 2014 02:17:31 PM
Old 07-10-2014
Quote:
Originally Posted by Corona688
I can see why it would cause issues when contiguous pages aren't available, but why using contiguous pages cause pages to become discontinuous?
Oracle doesn't technically request "contiguous" pages, it requests large pages. To get those large pages, the OS must coalesce smaller pages.

Oracle uses the large page, releases it, then something else requests normal size pages and the large page gets fragmented. Oracle comes back and requests larges pages...

Rinse, lather, repeat.

The coalescing necessary to create the large pages can have some nasty performance impacts as it tends to lock up virtual memory management while it's happening. And for any OS instance, VM management tends to get very single threaded when memory gets tight. Processes won't start or hang because fork() and brk() calls block while the virtual memory manager thrashes about.

This can be really bad with Oracle on Solaris using ZFS because the ZFS ARC cache uses lots of small pages and isn't exactly quick in letting them go, and the standard recommendation is "let the ARC cache get as big as it wants, it doesn't hurt anything." Umm, wrong.
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mincore(2)							System Calls Manual							mincore(2)

NAME
mincore() - determine residency of memory pages SYNOPSIS
DESCRIPTION
The function determines the residency of the memory pages specified by the range [addr, addr+len]. The status is returned as a character- per-page in the character array referenced by vec. The least significant bit of each character is set to 1 to indicate the referenced page is in memory and to 0 to indicate that it is not. The settings of other bits in each character are undefined and may contain other informa- tion about a page in future implementations. This system call returns the status of memory pages at page boundaries that encompass the range from addr to addr+len. The addr parameter is automatically rounded down to the nearest page size as returned by system call. If the len value is not a multiple of the page size as returned by the system call, the length of the range is rounded up to the next multiple of the page size. The vector vec must be large enough to contain this number of bytes: Warnings Because the status of a page can change between the time checks and returns the information, returned information might be outdated. Only locked pages are guaranteed to remain in memory. See mlock(2). RETURN VALUE
Upon successful completion, returns Otherwise, it returns and sets to indicate the error. ERRORS
The function will fail if: The vec argument points to an illegal address or an address for which the caller does not have write mode access. len argument has a value less than or equal to 0 or (addr+len) exceeds the architected virtual address space of the application. Addresses in the range [addr, addr+len] are invalid for the address space of a process, or one or more pages in the address range are not mapped. SEE ALSO
mmap(2), mlock(2), plock(2), sysconf(2), <sys/mman.h>. mincore(2)
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