From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Wilck Date: Fri, 10 Aug 2001 13:15:56 +0000 Subject: [Linux-ia64] ping(8) faster than light (do_get_fast_time) (fwd) Message-Id: List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org Hi, I have encountered the following strange problem that is related to the RedHat bug https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id7485 A "ping localhost" on RedHat 7.1 will generate the warning Warning: time of day goes back, taking countermeasures. Tracing ping(8) with tcpdump shows that echo request and echo reply packets have exactly the same time stamp. The "RELNOTES" file of the RedHat iputils package says * By Pekka Savola - SIOCGSTAMP/SO_TIMESTAMP are sensitive to bug in kernel. When get_fast_time != gettimeofday (f.e. timestampless x86), returned stamp can be out of sync with gettimeofday. Workaround is not to use SIOCGSTAMP/SO_TIMESTAMP on such systems. It appears that this is actually the reason for the above problem. The deeper reason is that on IA64, *do_get_fast_time() equals do_normal_gettime(), which simply returns xtime. On other architectures do_get_fast_time() is set to do_gettimeofday() in time_init(), which avoids the problem. 1) Is do_gettimeofday () too slow on IA64 to use it for do_get_fast_time() ? 2) If yes, would it be possible to susbtitute a do_get_fast_time() that is faster and less accurate than do_gettimeofday(), but still better than do_normal_gettime() ? 3) If no, the ping(8) program on IA64 should IMO be changed to not use SO_TIMESTAMP (equivalent to "ping -U" with the RH7.1 ping). Martin -- Martin Wilck FSC EP PS DS1, Paderborn Tel. +49 5251 8 15113