On Sat, May 08, 2010 at 05:24:00PM +0000, Tiago Marques wrote:
> I was just looking into this and the issue with SD cards' random write
> performance intrigued me in this particular case. Isn't recording
> being performed sequentially, hence no lag problem?
At the moment, assuming an application that opens a file, and writes to
it at _any_ speed, occasionally the write will block for a huge amount
of time.
As an example, consider this shell script:
#!/bin/sh
while true; do
echo ok
sleep 0.01
done
And then this command to execute it:
strace -o /tmp/k -e write -T /tmp/j > /root/file
And then after several minutes or hours of run the strace file is
analysed:
grep write k|cut -f2 -d'<'|cut -f1 -d'>'|sort -rn|more
One will find several instances of write(2) syscalls taking huge
amounts of time:
0.406725
0.347159
Now this is surprising given that the I/O rate is trivial, and that
there is ample memory for buffering the writes.
If the test is repeated with the writes directed to tmpfs, there are no
such delays.
If the test is repeated at a much faster I/O rate using dd(1), ensuring
that ample free memory is available to buffer the writes, then one still
gets huge delays. Consider this test sequence:
a. drop all caches,
b. verify free memory,
c. write 262144000 bytes in 8192 byte blocks,
d. verify cache is used,
e. look at the distribution of the slowest writes.
[r...@xo-a7-2a-5a tmp]# echo 1 > /proc/sys/vm/drop_caches
[r...@xo-a7-2a-5a tmp]# echo 2 > /proc/sys/vm/drop_caches
[r...@xo-a7-2a-5a tmp]# echo 3 > /proc/sys/vm/drop_caches
[r...@xo-a7-2a-5a tmp]# free
total used free shared buffers cached
Mem: 949256 96328 852928 0 88 30304
-/+ buffers/cache: 65936 883320
Swap: 0 0 0
[r...@xo-a7-2a-5a tmp]# time strace -o /tmp/k -e write -T dd if=/dev/zero
of=/root/file bs=8192 count=32000
32000+0 records in
32000+0 records out
262144000 bytes (262 MB) copied, 59.8669 s, 4.4 MB/s
real 0m59.929s
user 0m1.960s
sys 0m5.350s
[r...@xo-a7-2a-5a tmp]# free
total used free shared buffers cached
Mem: 949256 361192 588064 0 672 287568
-/+ buffers/cache: 72952 876304
Swap: 0 0 0
[r...@xo-a7-2a-5a tmp]# grep write k|cut -f2 -d'<'|cut -f1 -d'>'|sort -rn|more
11.648413
8.020875
4.299857
3.282466
2.960739
2.151048
2.147674
2.090824
1.972840
1.740923
1.721511
1.674147
1.652831
1.538634
1.430747
1.417739
1.000022
0.961700
0.847079
0.009241
0.009202
0.005332
I've no explanation for this behaviour.
I don't think any different filesystem will affect the issue, but it
would probably be a good thing to try. Let us know how it goes!
--
James Cameron
http://quozl.linux.org.au/
_______________________________________________
Devel mailing list
[email protected]
http://lists.laptop.org/listinfo/devel
