On 1/15/14, 12:00 AM, Claudio Freire wrote:
My completely unproven theory is that swapping is overwhelmed by
near-misses. Ie: a process touches a page, and before it's actually
swapped in, another process touches it too, blocking on the other
process' read. But the second process doesn't account for that page
when evaluating predictive models (ie: read-ahead), so the next I/O by
process 2 is unexpected to the kernel. Then the same with 1. Etc... In
essence, swap, by a fluke of its implementation, fails utterly to
predict the I/O pattern, and results in far sub-optimal reads.
Explicit I/O is free from that effect, all read calls are accountable,
and that makes a difference.
Maybe, if the kernel could be fixed in that respect, you could
consider mmap'd files as a suitable form of temporary storage. But
that would depend on the success and availability of such a fix/patch.
Another option is to consider some of the more "radical" ideas in this thread,
but only for temporary data. Our write sequencing and other needs are far less stringent
for this stuff.
--
Jim C. Nasby, Data Architect j...@nasby.net
512.569.9461 (cell) http://jim.nasby.net
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