On Mon, Nov 24, 2008 at 11:58 AM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > On Mon, Nov 24, 2008 at 7:29 AM, MinChan Kim <minchan.kim@xxxxxxxxx> wrote: >> I'm so late :) >> >> On Sun, Nov 23, 2008 at 4:17 PM, Wang Yu <wangyuict@xxxxxxxxx> wrote: >>> >>> >>> On Sun, Nov 23, 2008 at 2:23 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote: >>>> >>>> Peter Teoh wrote: >>>>> >>>>> when a process mmap() a section of a file into its own process memory, >>>>> the process memory will maintain a copy of the data of that section of >>>>> the file. >>>> >>>> No, it does not maintain a copy. >>>> >>>> It mmaps the page cache pages into its own address space. >>> >>> >>> According to your explanation, the flow is physical file(on disk)-->Page >>> Cache(on memory, but in kernel space)-->Process Memory(on memory, but in >>> user space). Is it? I am not sure.... >>>> >> >> Yes. When kernel find no page mapping, it allocate new page in page >> cache and copy from on-disk page to new page, then map the new page to >> user space address. >> so, Never duplication. >> >>>>> so...does there exists duplicated buffering? (one in kernel - >>>>> pagecache, and one in userspace - for mmap() content of the file in >>>>> process memory) >>>> >>>> No, there is no such double buffering. >>> >>> But what is the difference? Why linux do it? >> >> In case of read system call except O_DIRECT, It's duplication between >> user buffer and page cache. >> Read/write system call abstract page to file. so you always need user buffer. >> Let think. If you want read some data, First of all you need some >> space which is user buffer. >> Mmap system call abstract page to memory. so you can handle file as >> memory operation without user buffer that mean It don't have >> duplication overhead. > > I see. So u are saying that read() will duplicate buffer between > userspace (user buffer) and kernel (pagecache), but for mmap() > operation, since there is no duplication, all access from user process > will immediate trigger a context switch, to read in the data from > ring0, right? yeb. More exactly, to read in the data from file fault in page fault handler. > Since u normally read in blocks of data using read(), and access data > byte-wise with mmap()'s pointer, so read() is much more efficient, as > it trigger much lesser context switches than mmap()'s way of pointer > accessing kernel memory? > > Performance-wise, mmap() will perform worst off than read()? I am not sure which case is better. That's because read operation have a overhead memory copy from kernel to user. But it occur less page fault than mmap. but on-demand readahead or hint function can increase performance in case of sequential read Page fault VS memory copy ?? which is more overhead ?? I think Rik can answer this question. > Thanks. > > > -- > Regards, > Peter Teoh > > Ernest Hemingway - "Never mistake motion for action." > -- Kinds regards, MinChan Kim -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
