On 31 January 2016 at 15:09, Yannick Duch?ne <yannick_duchene at yahoo.fr> wrote:
> If it's memory mapped, it's less an efficiency issue, > Hm, can you elaborate on this assertion? I don't think I agree. Lets say sqlite wants to access a page in the DB/journal. In the case of normal file access this is a call to pread/ReadFile, and in the memory-mapped case a call to memcpy. Now, the data in question may or may not alreaby be in the OS's disk cache. If it is pread/ReadFile/memcpy proceeds without delay. If the data is not in the cache, pread/ReadFile blocks until the i/o is complete. Similarly, memcpy will encounter a page fault and the process will block until the OS completes the i/o required to fill the page in memory. I'll grant there's an extra syscall per i/o in the normal file access mode, but this cost is _vanishingly_ small compared to the time required to load data from disk. Have I misunderstood the mechanism behind memory-mapped file access? -Rowan
