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Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of
From: |
Jan Beulich |
Subject: |
Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of grants to be used |
Date: |
Mon, 14 May 2012 08:41:13 +0100 |
>>> On 11.05.12 at 19:07, Stefano Stabellini <address@hidden>
wrote:
> On Fri, 11 May 2012, Jan Beulich wrote:
>> >>> On 11.05.12 at 09:19, "Jan Beulich" <address@hidden> wrote:
>> > Legacy (non-pvops) gntdev drivers may require this to be done when the
>> > number of grants intended to be used simultaneously exceeds a certain
>> > driver specific default limit.
>> >
>> > Signed-off-by: Jan Beulich <address@hidden>
>> >
>> > --- a/hw/xen_disk.c
>> > +++ b/hw/xen_disk.c
>> > @@ -536,6 +536,10 @@ static void blk_alloc(struct XenDevice *
>> > if (xen_mode != XEN_EMULATE) {
>> > batch_maps = 1;
>> > }
>> > + if (xc_gnttab_set_max_grants(xendev->gnttabdev,
>> > + max_requests * BLKIF_MAX_SEGMENTS_PER_REQUEST + 1) < 0)
>>
>> In more extensive testing it appears that very rarely this value is still
>> too low:
>>
>> xen be: qdisk-768: can't map 11 grant refs (Cannot allocate memory, 342 maps)
>>
>> 342 + 11 = 353 > 352 = 32 * 11
>>
>> Could someone help out here? I first thought this might be due to
>> use_aio being non-zero, but ioreq_start() doesn't permit more than
>> max_requests struct ioreqs-s to be around.
>
> Actually 342 + 11 = 353, that should be still OK because it is equal to
> 32 * 11 + 1, where the additional 1 is for the ring, right?
The +1 is for the ring, yes. And the calculation in the driver actually
appears to be fine. It's rather an issue with fragmentation afaict -
the driver needs to allocate 11 contiguous slots, and such may not
be available. I'll send out a v2 of the patch soon, taking fragmentation
into account.
Jan