Date: Tue, 13 Oct 2009 16:25:34 +0200 From: Ivan Voras <ivoras@freebsd.org> To: freebsd-stable@freebsd.org Subject: Re: Extreme console latency during disk IO (8.0-RC1, previous releases also affected according to others) Message-ID: <hb22kn$m3b$1@ger.gmane.org> In-Reply-To: <B5BA3ACC-BC06-4042-8434-0D9395A0F478@freebsd.org> References: <E316139E-FFCF-432F-8DCE-62B120C38E55@exscape.org> <CC16B639-7A75-4016-A8A8-5C59E9CD5E95@exscape.org> <hb1qs0$qjd$1@ger.gmane.org> <alpine.BSF.2.00.0910131406340.26071@fledge.watson.org> <9bbcef730910130633w150571a0k461fb4e67a51fb1d@mail.gmail.com> <B5BA3ACC-BC06-4042-8434-0D9395A0F478@freebsd.org>
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Robert N. M. Watson wrote: > > On 13 Oct 2009, at 14:33, Ivan Voras wrote: > >>> If (1) is highly variable during I/O, it's almost certainly a >>> property of >>> the VM technology you're using, and there's nought to be done about >>> it in >>> the guest OS. >> >> Here's an example of a ping session with 0.1s resolution during a few >> seconds-stall in ssh: >> >> 64 bytes from 161.53.72.188: icmp_seq=1576 ttl=64 time=0.383 ms >> 64 bytes from 161.53.72.188: icmp_seq=1577 ttl=64 time=0.405 ms >> 64 bytes from 161.53.72.188: icmp_seq=1578 ttl=64 time=0.360 ms >> >> 64 bytes from 161.53.72.188: icmp_seq=2304 ttl=64 time=4.194 ms >> 64 bytes from 161.53.72.188: icmp_seq=2305 ttl=64 time=0.454 ms >> 64 bytes from 161.53.72.188: icmp_seq=2306 ttl=64 time=0.376 ms >> >> note huge packet loss. It looks like it's VM fault or something like it. > > It sounds like the VM is failing to execute the guest during certain > types of I/O. A bit of scheduler tracing in the host OS probably > wouldn't go amiss to confirm that the VM really is suspending the guest > at about the same time ICMP latency goes up. However, given the above I > think I you can reasonable assume that the 4ms jump you're seeing there > is due to global host OS/VM scheduling, and not FreeBSD scheduling. Btw. it's not a "4 ms jump" - there are 726 lost packets in between.
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