hmm Unfortunatly Linux is nt relevent to FreeBSD so we can't comment directly..
it is possible that the mmapped region is marked non-cachable, which migh tmake a difference. I have no idea where "memcpy_to_iovec" in Linux is copying to so it's hard to comment. julian On Thu, 1 Jul 1999, Zhihui Zhang wrote: > > A graduate student here implements a mmap() interface to a TCP/IP network > card. He notices that it takes much longer time to copy from mmapp()'ed > area to another user area than it takes to copy the same amount of data > from kernel space to user space. The students here have no idea why this > could be possible. I hope someone on this list can give us a hint. Below > is a part of his original email. He uses rdtsc instruction to do the > timing. > > ------------------------------------------------------------------------ > Well I have implemented a memory mapped interface for the user in Linux > using the DEC 21140 Tulip ethernet card. Thus the user has access to the > buffers, but when I did a memcpy from the RX buffer to the user variable, > it took an extraordinary amount of time, approx 70 microsec for 1460 > btyes... where as the original scheme takes 25 microsec for the same data > when it does a memcpy_to_iovec in tcp_recvmsg(). > > I am confused by this unexpected timings. More than 80% of the time is > spent doing the memcpy. > ----------------------------------------------------------------------- > > Thanks for your help. > > -------------------------------------------------- > Zhihui Zhang. Please visit http://www.freebsd.org > -------------------------------------------------- > > > > To Unsubscribe: send mail to majord...@freebsd.org > with "unsubscribe freebsd-hackers" in the body of the message > To Unsubscribe: send mail to majord...@freebsd.org with "unsubscribe freebsd-hackers" in the body of the message
