Performance optimization in av::fixup - use buffered IO, not mapped file

[Eric Blake]

On 12/12/2012 08:39 AM, Ryan Johnson wrote:

> Does gcc/ld/whatever know the final file size before the first write?

No, but does it need to?  posix_fallocate() does not change file
contents; it merely says that anywhere there was previously a hole must
now be guaranteed to be backed by disk.  So gcc would write the file as
usual, and then just before close()ing the fd, do a final
posix_fallocate(fd, 0, len) with len determined by the final file size.

> 
> You have to posix_fallocate the entire file before any write that might
> create a hole, because the sparse flag poisons the loader,

Is there really a flag stuck into the file when it becomes sparse?

> and persists
> even if all gaps are later filled. For example, if I invoke the
> following commands:
> 
> cp --sparse=always $(which emacs-nox) sparse
> cp --sparse=never $(which emacs-nox) dense
> for f in sparse dense; do echo $f; time ./$f -Q --batch --eval
> '(kill-emacs)'; done
> cp --sparse=never dense sparse
> for f in sparse dense; do echo $f; time ./$f -Q --batch --eval
> '(kill-emacs)'; done
> du dense sparse

This doesn't point to a flag in the file, so much as cached information
(the file system is remembering that 'sparse' used to be sparse, even if
it is no longer sparse).  But your point about a file being cached at
some point while it is sparse, even if it is later made non-sparse, is
interesting.

> 
> The relevant output is:
>> sparse
>> real    0m1.791s
>>
>> dense
>> real    0m0.606s
>>
>> sparse
>> real    0m3.158s
>>
>> dense
>> real    0m0.081s
>>
>> 16728   dense
>> 16768   sparse
> 
> Given that we're talking about cygwin-specific patches for emacs and
> binutils anyway, would it be better to add a cygwin-specific fcntl call
> that clears the file's sparse flag?

What flag is there to clear?  Your cp demonstration showed that even
when we do a byte-for-byte copy of every byte (and the file is
non-sparse), the file system cache remembers that it used to be sparse.
 How do we defeat that file system cache?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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