Date: Thu, 1 Jul 1999 16:24:02 -0700 (PDT) From: Julian Elischer <julian@whistle.com> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: reason for slow user-user memory copy Message-ID: <Pine.BSF.3.95.990701162153.13958N-100000@current1.whistle.com> In-Reply-To: <Pine.GSO.3.96.990701185753.3735A-100000@sol.cs.binghamton.edu>
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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 majordomo@FreeBSD.org > with "unsubscribe freebsd-hackers" in the body of the message > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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