[Openstack] Network speed issue

Rick Jones rick.jones2 at hp.com
Tue Dec 16 23:09:36 UTC 2014

> Both results show a really high bandwidth but I can't get that speed for
> file transfers.

Well, at least we can more or less exonerate networking :)  Though 
frankly I've always felt that virtio was better than an emulated NIC.

How much memory to you have in the VM receiving the 4 GB file being 
transferred?  One hypothesis, and it is only a hypothesis, is that the 
reason the file transfer goes quicker at the beginning (if I recall 
correctly) is the receiving VM is still just writing to filecache within 
the VM itself.  Once the number of ditry pages in the filecache gets to 
some limit (which I can never remember) a flushing daemon will kick-in 
and start pushing pages out to "disc" (well, what the VM thinks of as 
its disc, whatever you've setup there).  At that point there may still 
be something of a "race" between the arriving bytes of the file and the 
flushing and if there ends-up being enough memory consumed by the 
filecache, writes to the file will end-up getting blocked, waiting for 
space to free-up.  And that won't happen any faster than the flusher can 
get them out.

And then there can be the similar thing happening between the VM and the 
host, where in this part we can think of the VM as being akin to the scp 
receiving process.

happy benchmarking,

rick jones

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