netdev - Re: [Xen-devel] xen-netfront possibly rides the rocket too often

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Message-ID: <20140516103239.GF18551@zion.uk.xensource.com>
Date:	Fri, 16 May 2014 11:32:39 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Stefan Bader <stefan.bader@...onical.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Zoltan Kiss <zoltan.kiss@...rix.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] xen-netfront possibly rides the rocket too often

On Fri, May 16, 2014 at 12:17:48PM +0200, Stefan Bader wrote:
> On 16.05.2014 12:09, Stefan Bader wrote:
> > On 16.05.2014 11:48, Wei Liu wrote:
> >> On Thu, May 15, 2014 at 02:14:00PM +0200, Stefan Bader wrote:
> >> [...]
> >>>> Wei.
> >>>>
> >>> Reading more of the code I would agree. The definition of MAX_SKB_FRAGS (at
> >>> least now with compound pages) cannot be used in any way to derive the number of
> >>> 4k slots a transfer will require.
> >>>
> >>> Zoltan already commented on worst cases. Not sure it would get as bad as that or
> >>> "just" 16*4k frags all in the middle of compound pages. That would then end in
> >>> around 33 or 34 slots, depending on the header.
> >>>
> >>> Zoltan wrote:
> >>>> I think the worst case scenario is when every frag and the linear buffer contains 2 bytes,
> >>>> which are overlapping a page boundary (that's (17+1)*2=36 so far), plus 15 of
> >>> them have a 4k
> >>>> page in the middle of them, so, a 1+4096+1 byte buffer can span over 3 page.
> >>>> That's 51 individual pages.
> >>>
> >>> I cannot claim to really know what to expect worst case. Somewhat I was thinking
> >>> of a
> >>> worst case of (16+1)*2, which would be inconvenient enough.
> >>>
> >>> So without knowing exactly how to do it, but as Ian said it sounds best to come
> >>> up with some sort of exception coalescing in cases the slot count goes over 18
> >>> and we know the data size is below 64K.
> >>>
> >>
> >> I took a stab at it this morning and came up with this patch. Ran
> >> redis-benchmark, it seemed to fix that for me -- only saw one "failed to
> >> linearize skb" during
> >>
> >>   redis-benchmark -h XXX -d 1000 -t lrange
> >>
> >> And before this change, a lot of "rides rocket" were triggered.
> >>
> >> Thought?
> > 
> > It appears at least to me as something that nicely makes use of existing code. I
> > was wondering about what could or could not be used. Trying to get ones head
> > around the whole thing is kind of a lot to look at.
> > 
> > The change at least looks straight forward enough.
> 
> The only woe for me is that I am looking puzzled at the implementation of
> skb_linearize(). Somehow the data_len element decides whether a skb can be
> linearized and basically how much it tries to pull from the tail. It probably
> makes sense ... just not to me with not deep experience here.
> 

Data_len is the size of paged data (that's the size of data in frags).
If it pulls everything from frags to linear area then SKB is linearized.

Wei.

> -Stefan
> 


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