Re: [LTP] [PATCH v1] readahead02.c: Use fsync instead of sync

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello,

Cyril Hrubis <chrubis@suse.cz> writes:

> Hi!
>> > The motivation of this change is base on the https://github.com/linux-test-project/ltp/issues/972
>> > which give following suggestion:
>> > "As we run the test inside a loop device I guess that we can also 
>> > sync and drop caches just for the device, which should be faster 
>> > than syncing and dropping the whole system. Possibly we just need 
>> > to umount it and mount it again."
>> 
>> I see. Well unless Cyril can show that the test is actually failing
>> somewhere (or there is a strong logical argument this will cause a
>> failure). Then this task is still valid, but low priority IMO.
>
> We do sync more than needed here, since we are looking at the per device
> counters we have to sync just the device we mount for the test, so this
> is optimization for the case that the system has many dirty cases and
> will need seconds or a minute to write them to the pernament storage.
>
>> > But currently i can not find any API to sync and drop caches just 
>> > ONLY for device, so base my view just replace sync whole 
>> > system to single file also can make a small help.
>> 
>> If we don't have one or more concrete failures to focus on then we
>> really have to research whether fsync (or syncfs FYI) or unmounting the
>> device are the correct thing to do. They will all have subtly different
>> effects.
>
> Looking at the code closely I'm starting to think that the sync is not
> required at all. What we do in the test is that we create file and sync
> it to the external storage. Then we read it a few times and mesure
> differences in cache. As far as I can tell we just need to drop the page
> cache after we have read the file. What do you think?
>
> In any case I would avoid changing the test before the release, but it's
> certainly something we can look at after that.

I still think same as before. It may be valid to drop sync or whatever,
but it's just not important compared to actively failing tests.

-- 
Thank you,
Richard.

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