On Friday, November 22, 2002, at 10:07 PM, Dave Harris wrote:
If we want this serialization library to be useful also for data-intensive computing we need to make sure that there we do not incur huge performance penalties. One call to memcpy, copying megabytes of data will surely be much faster than millions of virtual function calls, each copying just a few bytes.In-Reply-To: <[EMAIL PROTECTED]> On Tue, 19 Nov 2002 08:09:13 +0100 Matthias Troyer ([EMAIL PROTECTED]) wrote:Well, it is a lot of new virtual functions - one per type, almost doublingThe only solution which comes to my mind is additional virtual functions for writing blocks of primitive types, which default to just calling the operator<< (>>) n times, but can be overridden by optimized functions for those archive types where optimized writes (load) are possible. Nothing else would need to be changed. What is objectionable in that approach?
the size of the archive interface. Is it worth it?
While I can accept your arguments I do not believe this will really help. Unless serialization of large collections of primitive types are efficient, nobody with that need will use the library and you thus cannot gain experience.
I am inclined to say it is, because I generally favour giving the archive
more information about what is going on. And efficiency matters,
especially for such a fundamental library as serialisation. It does not
lead to more work for serialisation users or for the archive implementers.
Still, it is a lot of new functions, and once added they can never be
taken away. It may be better to leave them out now, and add them in 12
months time, when we have had more experience with the library and its
cost.
My rationale for including them right away is that for many formats there are functions which implementors of an archive might use and which have optimized versions for large arrays. Examples are,
XDR for portable binary archives
PVM and MPI for message passing
fwrite and fread for writing native binary files
memcpy for writing into memory-based buffers
Essentially all archive formats except for text-based ones can profit from these optimized functions.
Matthias
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