Thanks Peter for the extra detail. I thought there may have been something more mysterious going on. But it sounds like it was just the semantics of the term "use".
Cheers Aaron On 18/02/2011, at 9:25 PM, Peter Schuller <peter.schul...@infidyne.com> wrote: >> main argument for using mmap() instead of standard I/O is the fact >> that reading entails just touching memory - in the case of the memory >> being resident, you just read it - you don't even take a page fault >> (so no overhead in entering the kernel and doing a semi-context >> switch). > > Oh and in the case of Java/Cassandra, as Jonathan clued me in on > earlier, there is also the issue that byte[] arrays are mandated to be > zeroed when allocated which causes overhead typically because there > has to be a loop[1] somewhere writing a bunch of zeroes in, that > you're then just going to replace immediately. Mapping a file has no > such implications as long as you read directly from the underlying > direct ByteBuffer. > > [1] Not *necessarily*; a JVM could theoretically do byte[] allocations > in such a way that it already knows the contents is zeroed, but it > would be highly dependent on the GC/memory management technique used > by the JVM whether this is practical. (It just occurred to me that > Azul should get this for 'free' in their GC. Wonder if that's true.) > > -- > / Peter Schuller
