Re: [PATCH 1/5] initramfs: move unnecessary memcmp from hot path

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On Wed, 22 Sep 2021 23:35:50 +0000, Al Viro wrote:

> On Wed, Sep 22, 2021 at 01:52:18PM +0200, David Disseldorp wrote:
> > do_header() is called for each cpio entry and first checks for "newc"
> > magic before parsing further. The magic check includes a special case
> > error message if POSIX.1 ASCII (cpio -H odc) magic is detected. This
> > special case POSIX.1 check needn't be done in the hot path, so move it
> > under the non-newc-magic error path.  
> 
> You keep refering to hot paths; do you have any data to support that
> assertion?

The code-path is run for every single initramfs entry on every single
boot. Calling that "hot" is reasonable IMO.

> How much does that series buy you on average, and what kind of dispersion
> do you get before and after it?

I withdrew the perf claims for this patch after you rightly pointed out
that my previous numbers weren't statistically significant. After doing
further checks I noticed that there was more to be gained via the
INITRAMFS_PRESERVE_MTIME changes. Numbers for extraction performance
with and without mtime preservation can be found in [PATCH 5/5] of this
series.

> I'm not saying I hate the patches themselves, but those references in commit
> messages ping my BS detectors every time I see them ;-/

I'll try harder to strip anything that might be considered buzzwordy in
future, sorry.

Cheers, David



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