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Date: Mon, 13 May 2013 10:58:22 -0400 From: David Oostdyk <daveo@...mit.edu> To: Eric Wong <normalperson@...t.net> CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk> Subject: Re: high-speed disk I/O is CPU-bound? On 05/10/13 20:19, Eric Wong wrote: > Cc-ing Jens > > David Oostdyk <daveo@...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. I was using cfq, but I gave noop and deadline a try and don't see any significant difference in my testing. Thanks for the suggestion! I had not thought to test this yet. >> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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