* Nick Piggin (npiggin@xxxxxxx) wrote: > On Wed, May 19, 2010 at 11:45:42AM -0400, Steven Rostedt wrote: > > On Wed, 2010-05-19 at 17:33 +0200, Miklos Szeredi wrote: > > > On Wed, 19 May 2010, Linus Torvalds wrote: > > > > Btw, since you apparently have a real case - is the "splice to file" > > > > always just an append? IOW, if I'm not right in assuming that the only > > > > sane thing people would reasonable care about is "append to a file", then > > > > holler now. > > > > > > Virtual machines might reasonably need this for splicing to a disk > > > image. > > > > This comes down to balancing speed and complexity. Perhaps a copy is > > fine in this case. > > > > I'm concerned about high speed tracing, where we are always just taking > > pages from the trace ring buffer and appending them to a file or sending > > them off to the network. The slower this is, the more likely you will > > lose events. > > > > If the "move only on append to file" is easy to implement, I would > > really like to see that happen. The speed of splicing a disk image for a > > virtual machine only impacts the patience of the user. The speed of > > splicing tracing output, impacts how much you can trace without losing > > events. > > It's not "easy" to implement :) What's your ring buffer look like? > Is it a normal user address which the kernel does copy_to_user()ish > things into? Or a mmapped special driver? > > If the latter, it get's even harder again. But either way if the > source pages just have to be regenerated anyway (eg. via page fault > on next access), then it might not even be worthwhile to do the > splice move. Steven and I use pages to which we write directly by using the page address from the linear memory mapping returned by page_address(). These pages have no other mapping. They are moved to the pipe, and then from the pipe to a file (or to the network). It's possibly the simplest scenario you could think of for splice(). Thanks, Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>