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

Ryan Johnson
Wed Dec 12 17:12:00 GMT 2012

On 12/12/2012 12:03 PM, Christopher Faylor wrote:
> On Tue, Dec 11, 2012 at 07:13:04PM -0800, Daniel Colascione wrote:
>> On 12/11/2012 5:06 PM, Daniel Colascione wrote:
>>> On 12/10/2012 7:51 PM, Daniel Colascione wrote:
>>>> The key to generating a binary that repros the problem is to unexec emacs, then
>>>> try to repro with that generated binary, not a copy of it.
>>> The real explanation is a lot simpler: the binary is sparse. When you create a
>>> file mapping object for a sparse file, Windows discards all cached pages for
>>> that file. It makes sense that compilers (and Emacs unexec) would create sparse
>>> files as they seek around inside their outputs.
>> Anyway, the binary is sparse because our linker produces sparse files.
>> Would the Cygwin developers accept this patch? With it, applications would need
>> to explicitly use ftruncate to make files sparse. Considering the horrible and
>> unexpected performance implications of sparse files, I don't think generating
>> them automatically from a sequence of seeks and writes is the right thing to do.
> I don't know if this was already done (don't see it in a quick glance at
> the archives) but, if this is just a simple case of executable files
> being sparse, it seems like an obvious optimization would be to just to
> do a, e.g.,
> cp --sparse=never -p foo.exe foo.exe.tmp
> mv foo.exe.tmp foo.exe
> Wouldn't that remove the sparseness and wouldn't you see astounding
> performance improvments as a result?
Nope. You'd have to rm foo.exe first.

Doing so fixes the problem nicely, though, as you suggest.
> I don't think we should be considering ripping code out of Cygwin
> without some actual data to back up claims.  Testing something like the
> above should make it easier to justify.
> I'm actually rather surprised that setup.exe's tar code would maintain an
> executable's sparseness.
Setup is fine. It's home-brew stuff that suffers, unless/until invoking 
`make install' copies the sparse file to its final destination, losing 
the sparse property along the way.

Personally, I'm still in shock that the loader barfs so badly over 
sparse files... normal reads via mmap and fread use the fs cache just fine.


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