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How to read different ints from a Bigarray?
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Date: 2009-10-29 (23:43)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] Re: How to read different ints from a Bigarray?
Gerd Stolpmann <gerd@gerd-stolpmann.de> writes:

> Am Donnerstag, den 29.10.2009, 21:40 +0100 schrieb Florian Weimer:
>> * Goswin von Brederlow:
>> > - The data is passed to libaio and needs to be kept alive and unmoved
>> >   as long as libaio knows it.
>> It also has to be aligned to a 512-byte boundary, so you can use
>> O_DIRECT.  Linux does not support truely asynchronous I/O without
>> O_DIRECT AFAIK, which rarely makes it worth the trouble.
> Right. There is also the question whether aio for regular files (i.e.
> files backed by page cache) is continued to be supported at all - it is
> well known that Linus Torvalds doesn't like it. It can happen that at
> some day aio will be restricted to block devices only.
> So I wouldn't use it for production code, but it is of course still an
> interesting interface.
> Gerd

Damn. That seems so stupid. Then writing asynchronous will only be
possible with creating a pot full of worker thread, each one writing
one chunk. So you get all those chunks in random order submitted to
the kernel, the kernel has to reorder them, fit them back together,
write them and then wake up the right thread for each piece
completed. So much extra work while libaio has all the data already in
perfect structures for the kernel.

And how will you do barriers when writing with threads? Wait for all
threads to complete every time you hit a barrier and thereby stalling
the pipeline?