Jeremy Fitzhardinge, on 12/19/2008 11:21 PM wrote:
[...]
As with your case, we can simply copy the page data if this mechanism
isn't available. But it would be nice if it were.
1. Add net_priv analog in struct sk_buff, not in struct page. But then
it would be required that all the pages in each skb must be from the
same originator, i.e. with the same net_priv. It is unpractical to
change all the operations with skb's to forbid merging them, if they
have different net_priv. I tried, but quickly gave up. There are too
many such places in very not obvious code pieces.
I think Rusty has a patch to put some kind of put notifier in struct
skb_shared_info, but I'm not sure of the details.
2. Have in iSCSI-SCST a hashed list to translate page to iSCSI cmd by a
simple search function. This approach was rejected, because to copy a
page a modern CPU needs using MMX about 1500 ticks.
Is that the cold cache timing?
Should be L2 cache hot, which is almost always the case if FILEIO is
used, because data are just copied from the page cache. Although,
frankly, at the moment I can't find from where I got that number..
It was observed,
that each page can be referenced by TCP during transmit about 20 times
or even more. So, if each search needs, say, 20 ticks, the overall
search time will be 20*20*2 (to get() and put()) = 800 ticks. So, this
approach would considerably worse performance-wise to the chosen
approach and provide not too much benefit.
Wouldn't you only need to do the lookup on the last put?
No, because you can't say which one is the last. E.g., a page can be
mmaped to another process, while it's being transmitted. So, the only
possible way is to track all gets and puts done by networking using some
external reference counting (net_ref_cnt in case if iscsi-scst).
Vlad
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