linux-ext4 - Re: [PATCHSET v2] ext4: removal of alloc_sem locks from block allocation paths

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Date:	Tue, 10 May 2011 20:43:44 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	"Amir G." <amir73il@...rs.sourceforge.net>
cc:	Lukas Czerner <lczerner@...hat.com>, tytso@....edu,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCHSET v2] ext4: removal of alloc_sem locks from block
 allocation paths

On Tue, 10 May 2011, Amir G. wrote:

> On Tue, May 10, 2011 at 4:48 PM, Lukas Czerner <lczerner@...hat.com> wrote:
> > On Thu, 24 Mar 2011, amir73il@...rs.sourceforge.net wrote:
> >
> >> The purpose of this patch set is the removal of grp->alloc_sem locks
> >> from block allocation paths.
> >>
> >> The resulting code is cleaner and should perform better in concurrent
> >> allocating tasks workloads.
> > Hi Amir,
> >
> > Do you have any performance numbers indicating performance improvement
> > in concurrent allocations ? The only point where I can see taking
> > write semaphore is in filesystem resize code. Or am I missing something ?
> 
> Yes, you are. down_write is also taken when initializing a block group
> buddy cache for the first time (NEED_INIT flag is set).
> Anyway, I do NOT have any performance number since this wasn't the purpose of
> this work. This work was done for snapshots, but I do think that as I wrote:
> 1. The resulting code is cleaner
> 2. Getting rid of an unneeded semaphore in allocation path can only do good
> to performance, but I certainly don't have the kind of high scalability testing
> setup to show the performance improvements if there are any.

Well everyone who wants to use that group has to wait for this
initialization anyway, right ?

Of course I know that the purpose of this was to ease your snapshot
work, but I do not see that stated anywhere in the description ;).
Anyway I was just curious what impact does this have on the
performance since you've mentioned that, if you do not have any I am ok
with that.

Btw, patches looks good to me, but I have not done _very_ deep review.

Thanks!
-Lukas

> 
> >
> > Thanks!
> > -Lukas
> >
> >>
> >> I ran several xfstests runs with these patches (4K and 1K block size).
> >> I tried several online resizes and verifyed that both in-core and on-disk
> >> group counters are correct.
> >>
> >> v2->v1:
> >> - fix silly bug in patch 4/5 that triggers BUG_ON(incore == NULL)
> >> - replace get_undo_access() with get_write_access()
> >> - ran xfstests with block size 1K (where 2 groups share a buddy page)
> >>
> >> [PATCH v2 1/5] ext4: move ext4_add_groupblocks() to mballoc.c
> >> [PATCH v2 2/5] ext4: implement ext4_add_groupblocks() by freeing blocks
> >> [PATCH v2 3/5] ext4: synchronize ext4_mb_init_group() with buddy page lock
> >> [PATCH v2 4/5] ext4: teach ext4_mb_init_cache() to skip uptodate buddy caches
> >> [PATCH v2 5/5] ext4: remove alloc_semp
> >>
> >> --
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> >>
> >
> > --
> >
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