Paa Paa wrote:
I'm using Linux 2.6.20.4. I noticed that I get lower SATA hard drive
throughput with 2.6.20.4 than with 2.6.19. The reason was that 2.6.20
enables NCQ by defauly (queue_depth = 31/32 instead of 0/32). Transfer
rate was measured using "hdparm -t":
With NCQ (queue_depth == 31): 50MB/s.
Without NCQ (queue_depth == 0): 60MB/s.
20% difference is quite a lot. This is with Intel ICH8R controller and
Western Digital WD1600YS hard disk in AHCI mode. I also used the next
command to cat-copy a biggish (540MB) file and time it:
rm temp && sync && time sh -c 'cat quite_big_file > temp && sync'
Here I noticed no differences at all with and without NCQ. The times
(real time) were basically the same in many successive runs. Around 19s.
Q: What conclusion can I make on "hdparm -t" results or can I make any
conclusions? Do I really have lower performance with NCQ or not? If I
do, is this because of my HD or because of kernel?
hdparm -t is a perfect example of a synthetic benchmark. NCQ was
designed to optimize real-world workloads. The overhead gets hidden
pretty well when there are multiple requests in flight simultaneously,
as tends to be the case when you have a user thread reading data while a
kernel thread is asynchronously flushing the user thread's buffered
writes. Given that you're breaking even with one user thread and one
kernel thread doing I/O, you'll probably get performance improvements
with higher thread counts.
-- Chris
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