Re: high-speed disk I/O is CPU-bound?

[Eric Wong <normalperson@yhbt.net>]

Cc-ing Jens

David Oostdyk <daveo@ll.mit.edu> wrote:
> Hello,
> 
> I have a few relatively high-end systems with hardware RAIDs which
> are being used for recording systems, and I'm trying to get a better
> understanding of contiguous write performance.
> 
> The hardware that I've tested with includes two high-end Intel
> E5-2600 and E5-4600 (~3GHz) series systems, as well as a slightly
> older Xeon 5600 system.  The JBODs include a 45x3.5" JBOD, a 28x3.5"
> JBOD (with either 7200RPM or 10kRPM SAS drives), and a 24x2.5" JBOD
> with 10kRPM drives.  I've tried LSI controllers (9285-8e, 9266-8i,
> as well as the integrated Intel LSI controllers) as well as Adaptec
> Series 7 RAID controllers (72405 and 71685).

Which I/O scheduler are you using?  noop (or deadline) may improve
things with hardware RAID.

> Normally I'll setup the RAIDs as RAID60 and format them as XFS, but
> the exact RAID level, filesystem type, and even RAID hardware don't
> seem to matter very much from my observations (but I'm willing to
> try any suggestions).  As a basic benchmark, I have an application
> that simply writes the same buffer (say, 128MB) to disk repeatedly.
> Alternatively you could use the "dd" utility.  (For these
> benchmarks, I set /proc/sys/vm/dirty_bytes to 512M or lower, since
> these systems have a lot of RAM.)
> 
> The basic observations are:
> 
> 1.  "single-threaded" writes, either a file on the mounted
> filesystem or with a "dd" to the raw RAID device, seem to be limited
> to 1200-1400MB/sec.  These numbers vary slightly based on whether
> TurboBoost is affecting the writing process or not.  "top" will show
> this process running at 100% CPU.
> 
> 2.  With two benchmarks running on the same device, I see aggregate
> write speeds of up to ~2.4GB/sec, which is closer to what I'd expect
> the drives of being able to deliver.  This can either be with two
> applications writing to separate files on the same mounted file
> system, or two separate "dd" applications writing to distinct
> locations on the raw device.  (Increasing the number of writers
> beyond two does not seem to increase aggregate performance; "top"
> will show both processes running at perhaps 80% CPU).
> 
> 3.  I haven't been able to find any tricks (lio_listio, multiple
> threads writing to distinct file offsets, etc) that seem to deliver
> higher write speeds when writing to a single file.  (This might be
> xfs-specific, though)
> 
> 4.  Cheap tricks like making a software RAID0 of two hardware RAID
> devices does not deliver any improved performance for
> single-threaded writes.  (Have not thoroughly tested this
> configuration fully with multiple writers, though.)
> 
> 5.  Similar hardware on Windows seems to be able to deliver >3GB/sec
> write speeds on a single-threaded writes, and the trick of making a
> software RAID0 of two hardware RAIDs does deliver increased write
> speeds.  (I only point this out to say that I think the hardware is
> not necessarily the bottleneck.)
> 
> The question is, is it possible that high-speed I/O to these
> hardware RAIDs could actually be CPU-bound above ~1400MB/sec?
> 
> It seems to be the only explanation of the benchmarks that I've been
> seeing, but I don't know where to start looking to really determine
> the bottleneck.  I'm certainly open to suggestions to running
> different configurations or benchmarks.
> 
> Thanks for any help/advice!
> Dave O.