From: David Vrabel <david.vrabel@citrix•com>
To: Wei Liu <wei.liu2@citrix•com>
Cc: <xen-devel@lists•xen.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle•com>,
Boris Ostrovsky <boris.ostrovsky@oracle•com>,
Ian Campbell <ian.campbell@citrix•com>, <netdev@vger•kernel.org>,
<msw@amazon•com>, <annie.li@oracle•com>
Subject: Re: [PATCH] xen-netback: count number required slots for an skb more carefully
Date: Thu, 5 Sep 2013 11:12:11 +0100 [thread overview]
Message-ID: <522858FB.807@citrix.com> (raw)
In-Reply-To: <20130904154400.GS14104@zion.uk.xensource.com>
On 04/09/13 16:44, Wei Liu wrote:
> On Wed, Sep 04, 2013 at 03:02:01PM +0100, David Vrabel wrote:
> [...]
>>>>
>>>> I think I prefer fixing the counting for backporting to stable kernels.
>>>
>>> The original patch has coding style change. Sans that contextual change
>>> it's not a very long patch.
>>
>> The size of the patch isn't the main concern for backport-ability. It's
>> the frontend visible changes and thus any (unexpected) impacts on
>> frontends -- this is especially important as only a small fraction of
>> frontends in use will be tested with these changes.
>>
>>>> Xi's approach of packing the ring differently is a change in frontend
>>>> visible behaviour and seems more risky. e.g., possible performance
>>>> impact so I would like to see some performance analysis of that approach.
>>>>
>>>
>>> With Xi's approach it is more efficient for backend to process. As we
>>> now use one less grant copy operation which means we copy the same
>>> amount of data with less grant ops.
>>
>> It think it uses more grant ops because the copies of the linear
>> portion are in chunks that do not cross source page boundaries.
>>
>> i.e., in netbk_gop_skb():
>>
>> data = skb->data;
>> while (data < skb_tail_pointer(skb)) {
>> unsigned int offset = offset_in_page(data);
>> unsigned int len = PAGE_SIZE - offset;
>> [...]
>>
>> It wasn't clear from the patch that this had been considered and that
>> any extra space needed in the grant op array was made available.
>>
>
> If I'm not mistaken the grant op array is already enormous. See the
> comment in struct xen_netbk for grant_copy_op. The case that a buffer
> straddles two slots was taken into consideration long ago -- that's
> why you don't see any comment or code change WRT that..
I'm not convinced that even that is enough for the current
implementation in the (very) worse case.
Consider a skb with 8 frags all 512 in length. The linear data will be
placed into 1 slot, and the frags will be packed into 1 slot so 9 grant
ops and 2 slots.
I definitely think we do not want to potentially regress any further in
this area.
David
next prev parent reply other threads:[~2013-09-05 10:12 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-03 17:29 [PATCH] xen-netback: count number required slots for an skb more carefully David Vrabel
2013-09-03 21:53 ` Wei Liu
2013-09-04 2:25 ` annie li
2013-09-04 6:04 ` Matt Wilson
2013-09-04 6:56 ` [Xen-devel] " annie li
2013-09-04 7:38 ` Wei Liu
2013-09-04 8:06 ` annie li
2013-09-04 8:20 ` Wei Liu
2013-09-04 8:36 ` Ian Campbell
2013-09-04 8:41 ` Wei Liu
2013-09-04 11:48 ` [PATCH] " David Vrabel
2013-09-04 12:41 ` Ian Campbell
2013-09-04 13:35 ` Wei Liu
2013-09-09 9:20 ` Wei Liu
2013-09-04 13:14 ` Wei Liu
2013-09-04 14:02 ` David Vrabel
2013-09-04 15:44 ` Wei Liu
2013-09-05 10:12 ` David Vrabel [this message]
2013-09-05 10:27 ` Wei Liu
2013-09-04 18:48 ` David Miller
2013-09-05 7:35 ` Ian Campbell
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