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Wed, 07 May 2008

Fast transactions in POHMELFS.

POHMELFS just switched to faster transactions allocated one-by-one with even smaller overhead (although it does not use kernel_sendpage() for page sending yet, it copies data).
System does not serialize after all transactions are completed (it waits after each one), but with new transaction allocation it is 1.5 times faster: 98MB/s vs. 64MB/s, note that without waiting for transaction completion it gets full wire speed of 125MB/s with 1500 byte MTU. And it is with highmem pages and thus slow kmap() of each one, and unmap after completion. I do not use ->sendpage() since it will force to split proper set of iovecs into mixed calls of kernel_sendmsg() and kernel_sendpage(), which I want to avoid so far. Now it is (again) faster than NFS, but I want to move further.
So, solution is rather trivial: wait until several transactions are completed. There is the whole infrastructure already there - in-flight transaction storage, per-transaction completion and destruction callbacks, proper reference counting and async completion.
Still only writing transactions are used (i.e. reading/lookup and others will not redirected to different servers).
There are some bugs of course, but that's the first development version after all.

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