定期的に現れるext4は遅いスレッド。今回はReiser3から移行したらカーネルコンパイルが
遅くなったというもの

Hello,



I'm considering replacing Reiser3 filesystem with some newer one.

I ran compilebench benchmark. Read performance is lower with ext4.

Is it expected or is it possible to fix it?



Ext4:

read tree total runs 11 avg 20.47 MB/s (user 0.53s sys 0.74s)

read compiled tree total runs 4 avg 32.94 MB/s (user 0.61s sys 1.17s)



Reiser3:

read tree total runs 11 avg 26.33 MB/s (user 0.54s sys 0.81s)

read compiled tree total runs 4 avg 41.82 MB/s (user 0.62s sys 1.36s)



RAID details:



md8 : active raid10 sda7[0] sdd7[3] sdc7[2] sdb7[1]

      62925824 blocks 256K chunks 2 far-copies [4/4] [UUUU]



Ext4:

mkfs.ext4 -E stride=64,stripe-width=128 /dev/md8

mount -t ext4 -o noatime,auto_da_alloc,commit=600 /dev/md8 /mnt/md8



Reiser3:

mount -t reiserfs /dev/md8 /mnt/md8

mount -t reiserfs -o noatime,notail /dev/md8 /dev/md8



Ext4 results:

intial create total runs 10 avg 172.76 MB/s (user 0.43s sys 0.60s)

create total runs 14 avg 36.49 MB/s (user 0.42s sys 0.59s)

patch total runs 15 avg 15.16 MB/s (user 0.24s sys 0.49s)

compile total runs 14 avg 64.07 MB/s (user 0.10s sys 0.59s)

clean total runs 10 avg 393.43 MB/s (user 0.02s sys 0.06s)

read tree total runs 11 avg 20.47 MB/s (user 0.53s sys 0.74s)

read compiled tree total runs 4 avg 32.94 MB/s (user 0.61s sys 1.17s)

delete tree total runs 10 avg 2.51 seconds (user 0.24s sys 0.42s)

delete compiled tree total runs 4 avg 2.63 seconds (user 0.28s sys 0.50s)

stat tree total runs 11 avg 1.99 seconds (user 0.23s sys 0.18s)

stat compiled tree total runs 7 avg 2.11 seconds (user 0.27s sys 0.21s)



Reiser3 results:

intial create total runs 10 avg 82.74 MB/s (user 0.45s sys 1.13s)

create total runs 14 avg 28.54 MB/s (user 0.45s sys 1.19s)

patch total runs 15 avg 10.91 MB/s (user 0.24s sys 0.86s)

compile total runs 14 avg 47.49 MB/s (user 0.10s sys 1.27s)

clean total runs 10 avg 270.21 MB/s (user 0.02s sys 0.15s)

read tree total runs 11 avg 26.33 MB/s (user 0.54s sys 0.81s)

read compiled tree total runs 4 avg 41.82 MB/s (user 0.62s sys 1.36s)

delete tree total runs 10 avg 3.38 seconds (user 0.24s sys 0.72s)

delete compiled tree total runs 4 avg 4.14 seconds (user 0.27s sys 0.88s)

stat tree total runs 11 avg 2.09 seconds (user 0.22s sys 0.18s)

stat compiled tree total runs 7 avg 2.27 seconds (user 0.25s sys 0.21s)

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creating tar of compiled kernel takes much longer with ext4



time tar c linux-2.6.31 | cat >/dev/null



reiser3:

real    0m11.425s

user    0m0.229s

sys     0m1.586s



ext4:

real    0m23.942s

user    0m0.183s

sys     0m1.440s



partition was remounted before test

これに対するTedの回答。
dir_indexをOFFにしたら速くなると思うよ。ext4の改善はない。

You didn't say which version of the kernel you are using, which could

be important when asking these sorts of questions about potential

performance problems.



However, in this case, I suspect the issue is the nature of how

compilebench is structured.  Compilebench does the following which

makes it work particularly well for filesystems like reiserfs and

btrfs, and not so much for ext3 and ext4.  Quoting from the

compilebench web page:



   compilebench starts by putting these lists of file names into an

   order native to the filesystem it is working on. The files are

   created in sorted order based on the filename, and then readdir is

   used to find the order the filesystem uses for storing the

   names. After this initial phase, the filesystem native order is

   used for creates, patches and compile. Deleting, reading and

   stating the trees are done in readdir order.



The key here is that it reads the tree in readdir order.  Normally,

when you compile a kernel, the order in which you read and write files

is controlled by the Makefile; you don't get to read and write the

files in the order which just happens to be the most convenient for

the file system's b-tree hash algorithm.



Now, there are some workloads which compilebench might accurately

model --- for example, tar'ing up a directory.  However, despite the

name of the benchmark, it doesn't accurately model a kernel compile.



If you only care about compilebench numbers, you can try creating the

file system with the dir_index feature disabled.  This is the feature

that speeds up random access to directories; unfortunately, it means

that when you read files in readdir order, it causes extra random

reads to th einode table.  However, if your real-life workload is one

where file reads are always magically in readdir order, dir_index adds

overhead without adding any benefit.   



The bottom line is that I'm not terribly worried about trying to

improve ext4's performance on compilebench, since I don't believe it's

a benchmark that models realistic real-life workloads.



Regards,



						- Ted