On Mon, Mar 12, 2007 at 03:20:12PM +0100, Jan Kara wrote: > Hi, > > > Hi, I am encountering a performance problem, which I have tracked into the > > Linux kernel. The problem occurs with my experimental web server that uses > > sendfile to repeatedly transmit files. The files are based on the static > > portion of the SPECweb99 fileset and range in size to model a reasonable > > workload. With this workload, a significant number of the requests are > > for files of size 4 KB or less. > > > > I have determined that the performance problems occurs in the function > > do_generic_mapping_read in file mm/filemap.c for kernel version 2.6.20.1. > > Here is the specific code fragment: > > > > /* > > * When (part of) the same page is read multiple times > > * in succession, only mark it as accessed the first time. > > */ > > if (prev_index != index) > > mark_page_accessed(page); > Actually, the code is like that certainly for two years :).
Did it always use ra->prev_page? ISTR it using pos%PAGE_SIZE == 0 at some stage (ie. read from the start of a page -- obviously that also has holes). > > The implication of this code is that for files of size less than or equal > > to a single page, the page associated with such a file is likely to get > > evicted from the cache regardless of how frequently it is accessed. The > > reason is that after the first access, prev_index is always zero and index > > can only be zero. Hence, mark_page_accessed is never called after the > > first time the file is requested. As a result, the page is evicted from > > the cache no matter how frequently it is used. By changing the kernel to > > always call mark_page_accessed for these files, the server throughput is > > increased by as much as 20%. > Your analysis seems to be right. But to observe this behaviour you have > to have the file open and just always reread it using the same file > descriptor, don't you? That's probably not too common... > > > I was wondering if anyone could explain why the call to mark_page_accessed > > is conditional? That is, what problem it is trying to solve. It would seem > > that in many scenarios, if the same page is accessed repeatedly, then it > > would be appropriate to keep that page cached. > I also don't know why the condition is there but it's there at least > for two years so I'm not sure anybody remembers ;). Nick, do you have > an idea? Yeah it is there because that is basically how our "use once" detection handles the case where an app does not read in chunks that are PAGE_SIZE multiples and PAGE_SIZE aligned. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
