I'm pretty sure that the tempfile penalty is unrelated to the results
Paul sees in his tests, at least for smaller files.

If the content-length header indicates that the body of the item is
smaller than 128k, it does not appear to be spooled from a tempfile at
all.  This also may be why there isn't much of a difference between
"normal" product results and Paul's "patched" product results at his 12K
file size as opposed to the roughly 60% difference in speed at at 324K
and 1216K (I cannot explain the large difference in results at 38322k
across products; it appears to be product-specific).  Also, for this
file size, wrapping the data in a producer is actually just more
overhead; products would be better off to do a direct RESPONSE.write if
the total file size was under 128k (as evidenced by the fact that the
iterator-based products are slower at small file sizes).

It's a bit of a mystery about why retrieving data over ZEO is an order
of magnitude slower than retrieving it from a local file storage when
the ZEO server is actually on the same physical machine.   I'm sure
there are lots of optimization strategies for making this faster that
involve a lot of heavy lifting and hard thinking in a dark room. ;-) 
While that might be fun, in the meantime, for the hoi-polloi who just
need to get stuff done, I believe that one simple answer to this is, as
usual, to do very aggressive caching and avoid ZODB access altogether
for large blobs.  The ZEO caching algorithm is largely agnostic about
what kinds of objects it caches and doesn't allow for setting a
cache-eviction policy other than whatever its implementors consider
optimal for the most general case.   This would be fairly difficult to
change in the best-case circumstance for someone with deep ZODB
knowledge.  A more effective strategy for "normal" people might be to
contribute to the "hardening" of something like FileCacheManager, which
prevents ZODB access in the first place and allows for fine-grained
policies on an application level by caching blobs on client filesystems.

- C

On Thu, 2004-04-08 at 12:16, Paul Winkler wrote:
> On Wed, Mar 24, 2004 at 01:32:18PM -0500, Shane Hathaway wrote:
> > Jeremy has suggested that object pre-fetching could be added to ZODB.
> This is much on my mind currently.
> Any thoughts on what an API for pre-fetching might look like?
> The use case that most concerns me is:
> If you have an Image or File object with a very long Pdata chain,
> you're likely to A) thrash the ZEO client cache if it's not big enough,
> and B) spend a looooong time waiting for all the objects in the chain
> to load.  At least, this is my theory on what's slowing things
> down - I will be trying to verify this today. See the bottom of this page:
> http://www.slinkp.com/code/zopestuff/blobnotes

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