[Bug 199727] CPU freezes in KVM guests during high IO load on host

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199727

Roland Kletzing (devzero@web.de) changed:

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                 CC|                            |devzero@web.de

--- Comment #1 from Roland Kletzing (devzero@web.de) ---
i can confirm there is a severe issue here, which renders kvm/proxmox virtually
unusable when you have significantly io loaded hosts, i.e. if there is lots of
write io on the host or guest.

whenever you get into a situation when the disk io where the vm resides on is
getting saturated, the VMs start going nuts and getting hiccup, i.e. severe
latency is getting added to the guests. 

they behave "jumpy", you can't use the console or they are totaly sluggish,
ping goes up above 10secs , kernel throws "BUG: soft lockup - CPU#X stuck for
XXs!" and such...

i have found that with cache=writeback for the virtual machines disk which
reside on the appropriate hdd wich heavy io, things go much much more smoothly. 

without cache=writeback , live migration/move could make a guest go crazy.

now with cache=writeback i could do 3 live migrations in parallel , even with
lots of io inside the virtual machines, and even with additional writer/reader
in the host os (dd from/to the disk - ping to the guests mostly is <5ms.

so, to me this problem appears to be related to disk io saturation and probably
related to sync writes, what else can explain that cache=writeback helps so
much ?

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