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Re: [Xen-devel] [PATCH 0/4] Implement persistent grant in xen-netfront/netback


On Thu, Nov 15, 2012 at 03:03:07PM +0800, Annie Li wrote:
> This patch implements persistent grants for xen-netfront/netback. This
> mechanism maintains page pools in netback/netfront, these page pools is used 
> to
> save grant pages which are mapped. This way improve performance which is 
> wasted
> when doing grant operations.
> Current netback/netfront does map/unmap grant operations frequently when
> transmitting/receiving packets, and grant operations costs much cpu clock. In
> this patch, netfront/netback maps grant pages when needed and then saves them
> into a page pool for future use. All these pages will be unmapped when
> removing/releasing the net device.

Do you have performance numbers available already? with/without persistent 

> In netfront, two pools are maintained for transmitting and receiving packets.
> When new grant pages are needed, the driver gets grant pages from this pool
> first. If no free grant page exists, it allocates new page, maps it and then
> saves it into the pool. The pool size for transmit/receive is exactly tx/rx
> ring size. The driver uses memcpy(not grantcopy) to copy data grant pages.
> Here, memcpy is copying the whole page size data. I tried to copy len size 
> data
> from offset, but network does not seem work well. I am trying to find the root
> cause now.
> In netback, it also maintains two page pools for tx/rx. When netback gets a
> request, it does a search first to find out whether the grant reference of
> this request is already mapped into its page pool. If the grant ref is mapped,
> the address of this mapped page is gotten and memcpy is used to copy data
> between grant pages. However, if the grant ref is not mapped, a new page is
> allocated, mapped with this grant ref, and then saved into page pool for
> future use. Similarly, memcpy replaces grant copy to copy data between grant
> pages. In this implementation, two arrays(gnttab_tx_vif,gnttab_rx_vif) are
> used to save vif pointer for every request because current netback is not
> per-vif based. This would be changed after implementing 1:1 model in netback.

Btw is xen-netback/xen-netfront multiqueue support something you're planning to 
implement aswell? 
multiqueue allows single vif scaling to multiple vcpus/cores.


-- Pasi

> This patch supports both persistent-grant and non persistent grant. A new
> xenstore key "feature-persistent-grants" is used to represent this feature.
> This patch is based on linux3.4-rc3. I hit netperf/netserver failure on
> linux latest version v3.7-rc1, v3.7-rc2 and v3.7-rc4. Not sure whether this
> netperf/netserver failure connects compound page commit in v3.7-rc1, but I did
> hit BUG_ON with debug patch from thread
> http://lists.xen.org/archives/html/xen-devel/2012-10/msg00893.html
> Annie Li (4):
>   xen/netback: implements persistent grant with one page pool.
>   xen/netback: Split one page pool into two(tx/rx) page pool.
>   Xen/netfront: Implement persistent grant in netfront.
>   fix code indent issue in xen-netfront.
>  drivers/net/xen-netback/common.h    |   24 ++-
>  drivers/net/xen-netback/interface.c |   26 +++
>  drivers/net/xen-netback/netback.c   |  215 ++++++++++++++++++--
>  drivers/net/xen-netback/xenbus.c    |   14 ++-
>  drivers/net/xen-netfront.c          |  378 
> +++++++++++++++++++++++++++++------
>  5 files changed, 570 insertions(+), 87 deletions(-)
> -- 
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> http://lists.xen.org/xen-devel

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