Re: [PATCH v2] vfs: avoid recopying file names in getname_flags

[Boqun Feng]

Hi Al,

On Sun, Apr 12, 2015 at 12:56:55AM +0100, Al Viro wrote:
> On Tue, Apr 07, 2015 at 04:38:26PM +0800, Boqun Feng wrote:
> > Ping again...
> 
> What exactly does it buy us?  You need a pathname just a bit under 4Kb, which,
> with all due respect, is an extremely rare case.  Resulting code is more
> complicated, we _still_ copy twice (sure, the second time is for 16 bytes or
> so, but...), instead of "compare with the address of embedded array" we get
> the loveliness like
> > > > +	if (name->name != ((char *)name - EMBEDDED_NAME_MAX)) {
> this...   _And_, on top of everything else, we get name and name->name
> guaranteed to hit different cachelines, in all cases, including the common
> ones.
> 
> What for?  It's not as if userland memory had been communicated with by
> IP over carrier pigeons, after all, and the cost of 4Kb worth of
> (essentially) memcpy() is going to be
> 	a) incurred in extremely rare case
> and
> 	b) be dwarfed by the work we need to _do_ with what we'd copied.
> After all, that pathname is going to be parsed and traversed - all 4Kb
> worth of it.
> 
> So what's the point?

Thank you for your response.

Well, my original purpose of doing this is to avoid recopying file
names, I thought although long file names are race, it's worthy if we
can optimize without affecting common cases. But you are right, I fail
to take cachelines into consideration, so comman cases are affected.

Before I totally give it up, I'd like to run some performance tests
about this patch, which I should do before sending the patch, I will do
better next time ;-)
If I find something new, I will let you know.

Thanks again for your comments.

Regards,
Boqun Feng

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