How to obtain good concurrent read performance from disk,
I’d like to ask a question then follow it up with my own answer, but also see what answers other people have.
We have two large files which we’d like to read from two separate threads concurrently. One thread will sequentially read fileA while the other thread will sequentially read fileB. There is no locking or communication between the threads, both are sequentially reading as fast as they can, and both are immediately discarding the data they read.
Our experience with this setup on Windows is very poor. The combined throughput of the two threads is in the order of 2-3 MiB/sec. The drive seems to be spending most of its time seeking backwards and forwards between the two files, presumably reading very little after each seek.
If we disable one of the threads and temporarily look at the performance of a single thread then we get much better bandwidth (~45 MiB/sec for this machine). So clearly the bad two-thread performance is an artefact of the OS disk scheduler.
Is there anything we can do to improve the concurrent thread read performance? Perhaps by using different APIs or by tweaking the OS disk scheduler parameters in some way.
The files are in the order of 2 GiB each on a machine with 2GiB of RAM. For the purpose of this question we consider them not to be cached and perfectly defragmented. We have used defrag tools and rebooted to ensure this is the case.
We are using no special APIs to read these files. The behaviour is repeatable across various bog-standard APIs such as Win32’s CreateFile, C’s fopen, C++’s std::ifstream, Java’s FileInputStream, etc.
Each thread spins in a loop making calls to the read function. We have varied the number of bytes requested from the API each iteration from values between 1KiB up to 128MiB. Varying this has had no effect, so clearly the amount the OS is physically reading after each disk seek is not dictated by this number. This is exactly what should be expected.
The dramatic difference between one-thread and two-thread performance is repeatable across Windows 2000, Windows XP (32-bit and 64-bit), Windows Server 2003, and also with and without hardware RAID5.
The problem seems to be in Windows I/O scheduling policy. According to what I found there are many ways for an O.S. to schedule disk requests. While Linux and others can choose between different policies, before Vista Windows was locked in a single policy: a FIFO queue, where all requests where splitted in 64 KB blocks. I believe that this policy is the cause for the problem you are experiencing: the scheduler will mix requests from the two threads, causing continuous seek between different areas of the disk.
Now, the good news is that according to and Vista introduced a smarter disk scheduler, where you can set the priority of your requests and also allocate a minimum badwidth for your process.
The bad news is that I found no way to change disk policy or buffers size in previous versions of Windows. Also, even if raising disk I/O priority of your process will boost the performance against the other processes, you still have the problems of your threads competing against each other.
What I can suggest is to modify your software by introducing a self-made disk access policy.
For example, you could use a policy like this in your thread B (similar for Thread A):
if THREAD A is reading from disk then wait for THREAD A to stop reading or wait for X ms Read for X ms (or Y MB) Stop reading and check status of thread A again
You could use semaphores for status checking or you could use perfmon counters to get the status of the actual disk queue.
The values of X and/or Y could also be auto-tuned by checking the actual trasfer rates and slowly modify them, thus maximizing the throughtput when the application runs on different machines and/or O.S. You could find that cache, memory or RAID levels affect them in a way or the other, but with auto-tuning you will always get the best performance in every scenario.
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