FT> As explained, this removes rmap (in-kernel cache) and use mmaped FT> buffer instead of read/write system calls for user/kernel FT> communication. Ok, I got your code to work, and I have run some benchmarks. I'll cut directly to the chase... I used dbench with a single process, for 120 seconds on a dm-userspace device mapping directly to an LVM device. I used my example.c and the example-rb.c provided with the ringbuffer version. The results are: with cache, chardev: 251 MB/s no cache, ringbuffer: 243 MB/s I am very pleased with these results. I assume that your code is not tuned for performance yet, which means we should be able to squeeze at least 8 MB/s more out to make it equal (or better). Even still, the amount of code it saves is worth the hit, IMHO. I do have a couple of comments: 1. You said that the ringbuffer saves the need for syscalls on each batch read. This is partially true, but you still use a write() to signal completion so that the kernel will read the u->k ringbuffer. So, at best, the number of syscalls made is half of my read()/write() method. I think it's possible that another signaling mechanism could be used, which would eliminate this call. I do think eliminating the copying with the ringbuffer approach is very nice; I like it a lot. 2. I was unable to get your code to perform well with multiple threads of dbench. While my code sustains performance with 16 threads, the non-cache/ringbuffer version slows to a crawl (~1MB/s with 16 procs). I noticed that the request list grows to over a 100,000 entries at times, which means that the response from userspace requires searching that linearly, which may be the issue. I am going to further study your changes, but I think in the end that I will incorporate most or all of them. Some work will need to be done to incorporate support for some of the newer features (endio, for example), but I'll start looking into that. Thanks a lot! -- Dan Smith IBM Linux Technology Center Open Hypervisor Team email: danms@xxxxxxxxxx
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