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SubjectRe: Thread implementations...
Linus Torvalds <> writes:

> I've also done some more testing of sendfile(), and the nice thing is that
> when I compared doing a file copy with sendfile compared to a plain "cp",
> the sendfile implementation was about twice as fast (at least my version
> of "cp" will just do read+write pairs over and over again). When I copied
> a 38MB file the "cp" took 1:58 seconds while sendfile took 1:08 seconds
> according to "time" (I have 512MB of RAM, so this was all cached,
> obviously)..

But how does it look if you compare it to something that uses mmap() ?
On my machine GNU cp always seems to use read/write for copying.

In this situation it would be nice to "redeclare" a mmaped region
to another fd. Currently it would have to do either mmap()->write()
[implicit copy] or mmap()->mmaped region[explicit copy], because one
mmaped data region can only belong to a specific inode.

Something like:
adr = mmap(0, length, PROT_READ, MAP_SHARED, input_fd, offset);
mmap(adr, length, PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_REUSE, output_fd, offset);
msync(adr, length, MS_INVALIDATE);

To copy data without it ever leaving the page cache. Do you think that is
possible, or are the fixed costs of mmap just too big to make it worthwhile?


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