Messages in this thread | | | Date | Tue, 03 Apr 2007 12:31:49 -0400 | From | Chris Snook <> | Subject | Re: Lower HD transfer rate with NCQ enabled? |
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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.
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