linux-ext4 - Re: ditching e4b->alloc

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Date:	Tue, 22 Feb 2011 22:02:39 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Ted Ts'o" <tytso@....edu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: ditching e4b->alloc_semp

On Tue, Feb 22, 2011 at 7:18 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> On 2011-02-21, at 1:02 PM, Amir Goldstein wrote:
>> On Mon, Feb 14, 2011 at 11:22 PM, Amir Goldstein <amir73il@...il.com> wrote:
>>> On Mon, Feb 14, 2011 at 9:58 PM, Ted Ts'o <tytso@....edu> wrote:
>>>
>>>> One thing that comes to mind about your question with the
>>>> e4b->alloc_semp causing problems.  If the only reason why we need it
>>>> is to protect against multiple attempts to initialize different block
>>>> groups that share the same buddy bitmap, can we solve the problem by
>>>> ditching e4b->alloc_semp entirely, and simply using lock_page() on the
>>>> buddy bitmap page to protect it?
>>>
>>> Perhaps. I imagine there is more than one elegant way to deal with that,
>>> but using a semaphore is not one of them.
>>> I will take a shot at evaporating e4b->alloc_semp.
>>
>> After looking at the code a bit, I find that the only critical resource
>> that several groups may share on a single page is the Uptodate flag,
>> which is used to indicate that the buddy cache for *all* these groups
>> is loaded and lock_page() and get_page() are used to protect it.
>>
>> There are 2 ways to eliminate this dependency:
>>
>> 1. (AKA easy lane) use a single page (or more) per block group.
>> this will increase the memory usage for 1K blocks fs and for 2K block fs
>> on 8K page system, but are these use cases really that common?
>
> I think some distros may use 1kB block filesystems for root, where there are lots of small files.  I wonder if smolt would have this kind of info?
>
>> 2. (AKA hard lane) attach buffer heads to buddy page and use
>> buffer_uptodate() and buffer_lock() instead of PageUptodate() and lock_page()
>> to initialize buddy cache of groups that share the same page.
>>
>> What do you say?
>> Shall I take easy lane?
>
> For flex_bg filesystems, it would probably make even more sense to just load all of the bitmaps for that page, since it won't waste any more memory or cause extra disk seeks.  I wonder what the memory vs. seek performance tradeoff is for 1k filesystems to load all the bitmaps even for the non-flex_bg case (i.e. would the second bitmap have been loaded anyway in most cases)?
>

I'm sorry. I don't follow. I see how disk seeks can be avoided if we
load all bitmaps of a flex_bg,
but there can be no more than 2 groups on a page (4 on 8k system).
So what do I gain? My goal  is to remove the locking protection on
allocations from different block groups.

Amir.
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