[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen-netback: count number required slots for an skb more carefully
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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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