Re: Degregated I/O Performance since 3.4 - Regression in 3.4?

[Tobias Geiger <tobias.geiger@vido.info>]

Am 23.04.2012 17:24, schrieb Konrad Rzeszutek Wilk:
> On Mon, Apr 23, 2012 at 12:53:03PM +0100, Stefano Stabellini wrote:
>> On Mon, 23 Apr 2012, Tobias Geiger wrote:
>>> Hello!
>>>
>>> i noticed a considerable drop in I/O Performance when using 3.4 (rc3 and rc4
>>> tested) as Dom0 Kernel;
>>>
>>> With 3.3 i get over 100mb/s in a HVM DomU (win64) with PV Drivers
>>> (gplpv_Vista2008x64_0.11.0.357.msi);
>>> With 3.4 it drops to about a third of that.
>>>
>>> Xen Version is xen-unstable:
>>> xen_changeset          : Tue Apr 17 19:13:52 2012 +0100 25209:e6b20ec1824c
>>>
>>> Disk config line is:
>>> disk = [ '/dev/vg_ssd/win7system,,hda' ]
>>> - it uses blkback.
>> I fail to see what could be the cause of the issue: nothing on the
>> blkback side should affect performances significantly.
>> You could try reverting the four patches to blkback that were applied
>> between 3.3 and 3.4-rc3 just to make sure it is not a blkback
>> regression:
>>
>> $ git shortlog v3.3..v3.4-rc3 drivers/block/xen-blkback
>> Daniel De Graaf (2):
>>        xen/blkback: use grant-table.c hypercall wrappers
> Hm.. Perhaps this patch fixes it a possible perf (I would think that
> the compiler would have kept the result of the first call to vaddr(req, i)
> somewhere.. but not sure) lost with the mentioned patch:
>
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index 73f196c..65dbadc 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -327,13 +327,15 @@ static void xen_blkbk_unmap(struct pending_req *req)
>   	int ret;
>
>   	for (i = 0; i<  req->nr_pages; i++) {
> +		unsigned long addr;
>   		handle = pending_handle(req, i);
>   		if (handle == BLKBACK_INVALID_HANDLE)
>   			continue;
> -		gnttab_set_unmap_op(&unmap[invcount], vaddr(req, i),
> +		addr = vaddr(req, i);
> +		gnttab_set_unmap_op(&unmap[invcount], addr,
>   				    GNTMAP_host_map, handle);
>   		pending_handle(req, i) = BLKBACK_INVALID_HANDLE;
> -		pages[invcount] = virt_to_page(vaddr(req, i));
> +		pages[invcount] = virt_to_page(addr);
>   		invcount++;
>   	}
>
>>        xen/blkback: Enable blkback on HVM guests
>>
>> Konrad Rzeszutek Wilk (2):
>>        xen/blkback: Squash the discard support for 'file' and 'phy' type.
>>        xen/blkback: Make optional features be really optional.
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org
>> http://lists.xen.org/xen-devel
that made it even worse :)
Write Performance is down to about 7mb/s (with 3.3: ~130mb/s)
Read "only" down to 40mb/s (with 3.3: ~140mb/s)

Greetings
Tobias