linux-ext4 - Re: [PATCH RFC] jbd: don't wake kjournald unnecessarily

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Message-ID: <50D1E16B.6000703@redhat.com>
Date:	Wed, 19 Dec 2012 09:46:51 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH RFC] jbd: don't wake kjournald unnecessarily

On 12/19/12 2:13 AM, Jan Kara wrote:
> On Tue 18-12-12 21:08:51, Eric Sandeen wrote:
>> On 12/18/12 8:05 PM, Jan Kara wrote:
>>> On Wed 19-12-12 02:27:10, Jan Kara wrote:
>>>>> With a u8 tid_t, the "else" clause from commit d9b0193 fires
>>>>> frequently; I really think the underlying problem is that tid_geq()
>>>>> etc does not properly handle wraparounds - if, say, target is 255
>>>>> and j_commit_request is 0, we don't know if j_commit_request
>>>>> is 255 tids behind, or 1 tid ahead.  I have to think about that
>>>>> some more, unless it's obvious to someone else.
>>>>   Well, there's no way to handle wraps better AFAICT. Tids eventually wrap
>>>> and if someone has stored away tid of a transaction he wants committed and
>>>> keeps it for a long time before using it, it can end up being anywhere
>>>> before / after current j_commit_request. The hope was that it takes long
>>>> enough to wrap around 32-bit tids. If this happens often in practice we may
>>>> have to switch to 64-bit tids (in memory, on disk 32-bit tids are enough
>>>> because of limited journal size).
>>
>> I was wondering if, since the tid_g*() functions only work if the
>> distance is half the unsigned int space, we can force a commit at some
>> point if j_transaction_sequence has gotten too far ahead?  I'm not sure
>> where or if that could be done...
>   I don't quiete understand. If someone stores tid = transaction->t_tid and
> in two weeks calls log_start_commit(tid), I don't see how any forcing of
> commits could solve that tid may now look ahead of the log...

I'm probably missing something, but I was thinking we could compare
j_commit_sequence to j_transaction_sequence and force a commit up to at least
j_commit_sequence if it's too "stale" - but I'm only handwaving. :)

-Eric

> 								Honza
> 

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