Re: [Xen-devel] Re : Re : Re : task btrfs-transacti:651 blocked for more than 120 seconds

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 02, 2017 at 04:32:39PM +0000, Olivier Bonvalet wrote:
> Le vendredi 29 septembre 2017 à 10:20 +0100, Roger Pau Monné a écrit :
> > On Thu, Sep 28, 2017 at 05:27:54PM +0000, Olivier Bonvalet wrote:
> > > Le jeudi 28 septembre 2017 à 17:12 +0100, Roger Pau Monné a écrit :
> > > > 
> > > > Quite likely. With so many PV block devices attached you either
> > > > have
> > > > to limit the number of queues and persistent grants per-device,
> > > > or
> > > > expand the number of grants allowed by Xen.
> > > > 
> > > > Can you try to set the following in the Xen command line [0] and
> > > > see
> > > > if that solves your issues:
> > > > 
> > > > gnttab_max_frames=64
> > > > 
> > > > Roger.
> > > > 
> > > > [0] http://xenbits.xenproject.org/docs/unstable/misc/xen-command-
> > > > line
> > > > .html
> > > > 
> > > > _______________________________________________
> > > > 
> > > 
> > > Oh, from the Novell's documentation [0] I read :
> > > 
> > > « General recommendation for determining the proper value for
> > > "gnttab_max_frames" is to multiply by 2 the number of attached
> > > disks. »
> > > 
> > > 
> > > Since I have about 250 RBD devices, I suppose I have to try
> > > directly
> > > with gnttab_max_frames=512, right ?
> > 
> > Do you have 250 devices attached to the same guest? If so I guess 512
> > might be sensible, although you said earlier that you had 26 devices
> > attached, not 250.
> > 
> > Roger.
> > 
> 
> Hi,
> 
> no VM have more than 26 devices. Except for the Dom0 which have about
> 300 devices to handle. Does dom0 affected by this gnttab_max_frames
> choice ?

No, this limit is only meaningful for DomUs, not for Dom0 (the disks
attached to Dom0 are not PV).

> Anyway, after booting each hypervisor with gnttab_max_frames=256 (or
> greater), it seems I don't reach this timeout anymore.

I think 256 is quite high, 64 should probably be enough. In any case
256 is 1MB of memory used by the grant table, which I guess is not
that bad.

Roger.

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