Re: Drive performance bottleneck

[Jens Axboe]

On Wed, Feb 02 2005, Ian Godin wrote:
> 
>   I am trying to get very fast disk drive performance and I am seeing 
> some interesting bottlenecks.  We are trying to get 800 MB/sec or more 
> (yes, that is megabytes per second).  We are currently using 
> PCI-Express with a 16 drive raid card (SATA drives).  We have achieved 
> that speed, but only through the SG (SCSI generic) driver.  This is 
> running the stock 2.6.10 kernel.  And the device is not mounted as a 
> file system.  I also set the read ahead size on the device to 16KB 
> (which speeds things up a lot):
> 
> blockdev --setra 16834 /dev/sdb
> 
> So here are the results:
> 
> $ time dd if=/dev/sdb of=/dev/null bs=64k count=1000000
> 1000000+0 records in
> 1000000+0 records out
> 0.27user 86.19system 2:40.68elapsed 53%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 0inputs+0outputs (0major+177minor)pagefaults 0swaps
> 
> 64k * 1000000 / 160.68 = 398.3 MB/sec
> 
> Using sg_dd just to make sure it works the same:
> 
> $ time sg_dd if=/dev/sdb of=/dev/null bs=64k count=1000000
> 1000000+0 records in
> 1000000+0 records out
> 0.05user 144.27system 2:41.55elapsed 89%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 0inputs+0outputs (17major+5375minor)pagefaults 0swaps
> 
>   Pretty much the same speed.  Now using the SG device (sg1 is tied to 
> sdb):
> 
> $ time sg_dd if=/dev/sg1 of=/dev/null bs=64k count=1000000
> Reducing read to 16 blocks per loop
> 1000000+0 records in
> 1000000+0 records out
> 0.22user 66.21system 1:10.23elapsed 94%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 0inputs+0outputs (0major+2327minor)pagefaults 0swaps
> 
> 64k * 1000000 / 70.23 = 911.3 MB/sec
> 
>   Now that's more like the speeds we expected.  I understand that the 
> SG device uses direct I/O and/or mmap memory from the kernel.  What I 
> cannot believe is that there is that much overhead in going through the 
> page buffer/cache system in Linux.

It's not going through the page cache that is the problem, it's the
copying to user space. Have you tried using O_DIRECT? What kind of
speeds are you getting with that?

-- 
Jens Axboe

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