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Subject: Re: [boost] [AFIO] Formal review
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2015-09-02 08:20:22

> > Can you describe in more detail exactly why this information is
> needed/how
> > it is used?
> This is a complex and lengthy answer if answered in full, and is a
> large part of the AFIO value add. The housekeeping done during handle
> open is used throughout AFIO to skip work in later operations under
> the assumption that handle opens are not common.
> Limiting the discussion to just the problem of race free filesystem
> on POSIX, imagine the problem of opening the sibling of a file in a
> directory whose path is constantly changing. POSIX does not provide
> an API for opening the parent directory of an open file descriptor.
> To work around this, you must first get the canonical current path of
> the open file descriptor, strip off the end, open that directory and
> then use that as a base directory for opening a file with the same
> leafname as your original file. You then loop all that if you fail to
> open that leafname, or the leafname opened has a different inode.
> Once you have the correct parent directory, you can open the sibling.
> This is an example of where caching the stat_t of handle during open
> saves syscalls and branches in more performance important APIs later
> on.

Sorry for asking a question again (and I'm sure Niall will ignore it because
he judges it as 'disrespectful, petty, or juvenile', but perhaps somebody
else can answer this):

In another mail you said AFIO wouldn't cache anything, thus must do a lot of
syscalls over and over again, which is the reason for it to be slow.
How does that relate to what you said above? Sorry if I misunderstood
something - again.

Regards Hartmut

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