Did a quick test on an ext4 partition. This is how it looks like:
debian:~/miklos# ./miklos_test /mnt/file
begin 1200662360 1200662360 1200662353
write 1200662361 1200662361 1200662353
mmap 1200662361 1200662361 1200662362
b 1200662363 1200662363 1200662362
msync b 1200662363 1200662363 1200662362
c 1200662365 1200662365 1200662362
msync c 1200662365 1200662365 1200662362
d 1200662367 1200662367 1200662362
munmap 1200662367 1200662367 1200662362
close 1200662367 1200662367 1200662362
sync 1200662367 1200662367 1200662362
debian:~/miklos# mount | grep /mnt
/root/image.ext4 on /mnt type ext4dev (rw,loop=/dev/loop0)
Following are the results of the measurements. Here are the relevant
portions of the test program:
#define FILE_SIZE (1024 * 1024 * 512)
p = mmap(0, FILE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
/* Bring the pages in */
for (i = 0; i < FILE_SIZE; i += 4096)
tmp = p[i];
/* Dirty the pages */
for (i = 0; i < FILE_SIZE; i += 4096)
p[i] = i;
/* How long did we spend in msync(MS_ASYNC)? */
gettimeofday(&tv_start, NULL);
msync(p, FILE_SIZE, MS_ASYNC);
gettimeofday(&tv_stop, NULL);
<<<
For reference tests, the following platforms were used:
1. HP-UX B.11.31, PA-RISC 8800 processor (800 MHz, 64 MB), Memory: 4 GB.
2. HP-UX B.11.31, 2 Intel(R) Itanium 2 9000 series processors (1.59 GHz, 18 MB),
Memory: 15.98 GB.
3. FreeBSD 6.2-RELEASE, Intel(R) Pentium(R) III CPU family 1400MHz, 2 CPUs.
Memory: 4G.
The tests of my solution were performed using the following platform:
A KVM x86_64 guest OS, current Git kernel. Host system: Intel(R) Core(TM)2
Duo CPU T7300 @ 2.00GHz. Further referred to as "the first case".
The following tables give the time difference between the two calls
to gettimeofday(). The test program was run three times in a raw
with a delay of one second between consecutive runs. On Linux
systems, the following commands were issued prior to running the
tests:
echo 80 >/proc/sys/vm/dirty_ratio
echo 80 >/proc/sys/vm/dirty_background_ratio
echo 30000 >/proc/sys/vm/dirty_expire_centisecs
sync
Table 1. Reference platforms.
------------------------------------------------------------
| | HP-UX/PA-RISC | HP-UX/Itanium | FreeBSD |
------------------------------------------------------------
| First run | 263405 usec | 202283 usec | 90 SECONDS |
------------------------------------------------------------
| Second run | 262253 usec | 172837 usec | 90 SECONDS |
------------------------------------------------------------
| Third run | 238465 usec | 238465 usec | 90 SECONDS |
------------------------------------------------------------
It looks like FreeBSD is a clear outsider here. Note that FreeBSD
showed an almost liner depencence of the time spent in the
msync(MS_ASYNC) call on the file size.
Table 2. The Qemu system. File size is 512M.
---------------------------------------------------
| | Before the patch | After the patch |
---------------------------------------------------
| First run | 35 usec | 5852 usec |
---------------------------------------------------
| Second run | 35 usec | 4444 usec |
---------------------------------------------------
| Third run | 35 usec | 6330 usec |
---------------------------------------------------
I think that the data above prove the viability of the solution I
presented. Also, I guess that this bug fix is most probably ready
for getting upstream.
Please apply the sixth version of my solution.
--
