Re: Linux 2.6.29

[Jeff Garzik <jeff@garzik.org>]

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Linus Torvalds wrote:
> 
> On Thu, 2 Apr 2009, Jeff Garzik wrote:
>> The most interesting thing I found:  the SSD does 80 MB/s for the first ~1 GB
>> or so, then slows down dramatically.  After ~2GB, it is down to 32 MB/s.
>> After ~4GB, it reaches a steady speed around 23 MB/s.
> 
> Are you sure that isn't an effect of double and triple indirect blocks 
> etc? The metadata updates get more complex for the deeper indirect blocks.
> 
> Or just our page cache lookup? Maybe our radix tree thing hits something 
> stupid. Although it sure shouldn't be _that_ noticeable.
> 
>> There is a similar performance fall-off for the Seagate, but much less
>> pronounced:
>> 	After 1GB:	52 MB/s
>> 	After 2GB:	44 MB/s
>> 	After 3GB:	steady state
> 
> That would seem to indicate that it's something else than the disk speed. 

Attached are some additional tests using sync_file_range, dd, an SSD and 
a normal SATA disk.  The test program -- overwrite.c -- is unchanged 
from my last posting, basically the same as Linus's except with 
posix_fadvise()

Observations:

* the no-name SSD does seem to burst the first ~1GB of writes rapidly, 
but degrades to a much lower sustained level, as observed before. 
Repeated tests do not produce ~80 MB/s, only the first test, which lends 
credence to the theory about background activity.

* For the SSD, overwrite is noticeably faster than dd.

* For the Seagate NCQ hard drive, dd is noticeably faster than overwrite.

* fadvise() appears to help, but mostly the results are either 
inconclusive or lost in the noise:  A slight increase in throughput, and 
a slight increase in system time.

The test sequence for both SATA devices was the following:

	3 x dd
	3 x overwrite
	3 x overwrite w/ fadvise(don't need)

System setup: Intel Nahalem(sp?) x86-64, ICH10, Fedora 10, ext3 
filesystem (mounted defaults + noatime), 2.6.29 vanilla kernel.

Regards,

	Jeff






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=======================================================
128GB, 3.0 Gbps no-name SATA SSD, x86-64, ext3, 2.6.29 vanilla

First dd(1) creates the file, others simply rewrite it.
=======================================================
24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 917.599 s, 27.4 MB/s)

real	15m30.928s
user	0m0.016s
sys	1m3.924s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 1056.92 s, 23.8 MB/s)

real	18m1.686s
user	0m0.016s
sys	1m4.816s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 1044.25 s, 24.1 MB/s)

real	17m37.884s
user	0m0.020s
sys	1m4.300s


writing 2800 buffers of size 8m
21.867 GB written in 645.56 (34 MB/s)

real	10m46.502s
user	0m0.044s
sys	0m35.990s


writing 2800 buffers of size 8m
21.867 GB written in 634.55 (35 MB/s)

real	10m35.448s
user	0m0.036s
sys	0m36.466s


writing 2800 buffers of size 8m
21.867 GB written in 642.00 (34 MB/s)

real	10m42.890s
user	0m0.044s
sys	0m34.930s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 639.49 (35 MB/s)

real	10m40.384s
user	0m0.036s
sys	0m38.582s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 636.17 (35 MB/s)

real	10m37.061s
user	0m0.024s
sys	0m39.146s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 636.07 (35 MB/s)

real	10m37.003s
user	0m0.060s
sys	0m39.174s


=======================================================
500GB, 3.0Gbps Seagate SATA drive, x86-64, ext3, 2.6.29 vanilla

First dd(1) creates the file, others simply rewrite it.
=======================================================
24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 494.797 s, 50.9 MB/s)

real	8m42.680s
user	0m0.016s
sys	0m58.176s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 498.295 s, 50.5 MB/s)

real	8m27.505s
user	0m0.016s
sys	0m58.744s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 492.145 s, 51.1 MB/s)

real	8m23.616s
user	0m0.016s
sys	0m59.064s


writing 2800 buffers of size 8m
21.867 GB written in 478.41 (46 MB/s)

real	7m59.690s
user	0m0.032s
sys	0m33.210s


writing 2800 buffers of size 8m
21.867 GB written in 513.54 (43 MB/s)

real	8m34.461s
user	0m0.048s
sys	0m33.342s


writing 2800 buffers of size 8m
21.867 GB written in 471.38 (47 MB/s)

real	7m52.641s
user	0m0.020s
sys	0m33.486s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 467.67 (47 MB/s)

real	7m48.756s
user	0m0.048s
sys	0m36.838s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 462.69 (48 MB/s)

real	7m43.597s
user	0m0.020s
sys	0m37.462s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 463.56 (48 MB/s)

real	7m44.472s
user	0m0.036s
sys	0m37.342s



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