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RE: [Xen-devel] more profiling

To: "Santos, Jose Renato G" <joserenato.santos@xxxxxx>, "Andy Grover" <andy.grover@xxxxxxxxxx>
Subject: RE: [Xen-devel] more profiling
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Sat, 1 Mar 2008 09:25:42 +1100
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Thread-topic: [Xen-devel] more profiling
> James,
> Could you please provide me some context and details of this work.
> This seems related to the work we are doing in netchannel2 to reuse
> grants, but I don't think I understand what is that you are trying to
> and how it is related.

The solution I ended up implementing was to keep a list of pre-allocated
pre-granted pages. Any time we need a new page (either for putting on
the rx list, or for copying a tx packet to) it comes from the list. If
there are no page on the list, a new page is allocated and granted. When
we are finished with the page, it goes back on the free list.

I'll also be writing some sort of garbage collector which runs
periodically (maybe every x seconds, or every x calls to
'put_page_on_freelist'). If during that interval the number of free
pages has been constantly above some threshold (32?), then we will
ungrant and free half the pages on the list. This will keep memory usage
reasonable while keeping performance good.

In the tx path, the windows xennet driver currently takes the sg list of
buffers per packet and copies them to a single page buffer. At first I
thought there would be some performance to be had in just passing the
backend the list of pages, but it looks like the memory copy operation
is much less expensive than the grant operation.


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