[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 0/4] Implement persistent grant in xen-netfront/netback
On Fri, 2012-11-16 at 11:37 +0000, ANNIE LI wrote: > > On 2012-11-16 17:57, Ian Campbell wrote: > > On Thu, 2012-11-15 at 07:03 +0000, Annie Li wrote: > >> This patch implements persistent grants for xen-netfront/netback. > > Hang on a sec. It has just occurred to me that netfront/netback in the > > current mainline kernels don't currently use grant maps at all, they use > > grant copy on both the tx and rx paths. > > Ah, this patch is based on v3.4-rc3. Nothing has changed in more recent kernels in this regard. > > > > The supposed benefit of persistent grants is to avoid the TLB shootdowns > > on grant unmap, but in the current code there should be exactly zero of > > those. > > Is there any performance document about current grant copy code in > mainline kernel? Not AFAIK. > > If I understand correctly this patch goes from using grant copy > > operations to persistently mapping frames and then using memcpy on those > > buffers to copy in/out to local buffers. I'm finding it hard to think of > > a reason why this should perform any better, do you have a theory which > > explains it? > > This patch is aiming to fix spin lock issue of grant operations, it > comes out to avoid possible grant operations(including grant map and copy). Makes sense. This is the sort of thing which ought to feature prominently in commit messages and/or introductory mails. > > Do you know > > that they both benefit from this change (rather than for example an > > improvement in one direction masking a regression in the other). > > On theory, this implementation avoid spinlock issue of grant operation, > so they should both benefit from it. It seems like having netfront simply allocate itself a pool of grant references which it reuses would give equivalent benefits whilst being a smaller patch, with no protocol change and avoiding double copying. In fact by avoiding the double copy I'd expect it to be even better. > > Were > > the numbers you previously posted in one particular direction or did you > > measure both? > > One particular direction, one runs as server, the other runs as client. I think you need to measure both dom0->domU and domU->dom0 to get the full picture since AIUI netperf sends the bulk data in only one direction with just ACKs coming back the other way. Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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