Re: slow live magration / xc_restore on xen4 pvops

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 02/06/2010 17:18, "Ian Jackson" <Ian.Jackson@eu.citrix.com> wrote:

> Andreas Olsowski writes ("[Xen-devel] slow live magration / xc_restore on xen4
> pvops"):
>> [2010-06-01 21:20:57 5211] INFO (XendCheckpoint:423) ERROR Internal
>> error: Error when reading batch size
>> [2010-06-01 21:20:57 5211] INFO (XendCheckpoint:423) ERROR Internal
>> error: error when buffering batch, finishing
> 
> These errors, and the slowness of migrations, are caused by changes
> made to support Remus.  Previously, a migration would be regarded as
> complete as soon as the final information including CPU states was
> received at the migration target.  xc_domain_restore would return
> immediately at that point.

This probably needs someone with Remus knowledge to take a look, to keep all
cases working correctly. I'll Cc Brendan. It'd be good to get this fixed for
a 4.0.1 in a few weeks.

 -- Keir

> Since the Remus patches, xc_domain_restore waits until it gets an IO
> error, and also has a very short timeout which induces IO errors if
> nothing is received if there is no timeout.  This is correct in the
> Remus case but wrong in the normal case.
> 
> The code should be changed so that xc_domain_restore
>  (a) takes an explicit parameter for the IO timeout, which
>      should default to something much longer than the 100ms or so of
>      the Remus case, and
>  (b) gets told whether
>     (i) it should return immediately after receiving the "tail"
>         which contains the CPU state; or
>     (ii) it should attempt to keep reading after receiving the "tail"
>         and only return when the connection fails.
> 
> In the case (b)(i), which should be the usual case, the behaviour
> should be that which we would get if changeset 20406:0f893b8f7c15 was
> reverted.  The offending code is mostly this, from 20406:
> 
> +    // DPRINTF("Buffered checkpoint\n");
> +
> +    if ( pagebuf_get(&pagebuf, io_fd, xc_handle, dom) ) {
> +        ERROR("error when buffering batch, finishing\n");
> +        goto finish;
> +    }
> +    memset(&tmptail, 0, sizeof(tmptail));
> +    if ( buffer_tail(&tmptail, io_fd, max_vcpu_id, vcpumap,
> +                     ext_vcpucontext) < 0 ) {
> +        ERROR ("error buffering image tail, finishing");
> +        goto finish;
> +    }
> +    tailbuf_free(&tailbuf);
> +    memcpy(&tailbuf, &tmptail, sizeof(tailbuf));
> +
> +    goto loadpages;
> +
> +  finish:
> 
> Ian.
> 
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