From: Al Viro > Sent: 21 September 2020 16:30 > > On Mon, Sep 21, 2020 at 03:21:35PM +0000, David Laight wrote: > > > You really don't want to be looping through the array twice. > > Profiles, please. I did some profiling of send() v sendmsg() much earlier in the year. I can't remember the exact details but the extra cost of sendmsg() is far more than you might expect. (I was timing sending fully built IPv4 UDP packets using a raw socket.) About half the difference does away if you change the copy_from_user() to __copy_from_user() when reading the struct msghdr and iov[] from userspace (user copy hardening is expensive). The rest is just code path, my gut feeling is that a lot of that is in import_iovec(). Remember semdmsg() is likely to be called with an iov count of 1. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
