linux-ext4 - Re: What represent 646345728 bytes

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Message-ID: <4B67577F.70207@redhat.com>
Date:	Mon, 01 Feb 2010 16:36:47 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	paul.chavent@...c.net
CC:	linux-ext4@...r.kernel.org
Subject: Re: What represent 646345728 bytes

Eric Sandeen wrote:

> So uninit_bg doesn't seem to help.  (this was on 2.6.32-ish)
> 
> some oprofiling may be in order ...

Or stapping ... thanks to hints from the stap guys, using a modified version of
http://sourceware.org/systemtap/examples/profiling/fntimes.stp
I printed out function times higher than avg and correlated to writes
above average from the testcase (cleaned up manually a bit):

1265062212927230 function ext4_mb_load_buddy well over average time (42303 vs 2)
1265062212927399 function ext4_mb_regular_allocator well over average time (42476 vs 5495)
1265062212949252 function ext4_mb_mark_diskspace_used well over average time (21739 vs 196)
1265062212949306 function ext4_mb_new_blocks well over average time (64738 vs 2457)
1265062212949647 function ext4_ext_get_blocks well over average time (65211 vs 1106)
1265062212949678 function ext4_get_blocks well over average time (65357 vs 1844)
1265062212949695 function ext4_get_block well over average time (65479 vs 683)
1265062212951284 function ext4_ind_direct_IO well over average time (68891 vs 3034)
1265062212951299 function ext4_direct_IO well over average time (68908 vs 3046)
1265062212951497 function ext4_file_write well over average time (69437 vs 3590)

1265062212951534 size 134470144 time 69477310 avg 3601820

Unfortunately under ext4_mb_load_buddy is a lot of static/inlined functions
so no more detailed info yet.

But loading the buddy bitmap for a new group seems to be the big
hitter here - I'll keep digging, or maybe Aneesh, who groks mballoc
better than I do (I think) might have an idea.

But yeah, writes randomly taking 20x longer than usual sounds bad.  :(

-Eric
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