2018.01.16 00:24 "Re: [Tiff] Strategies for multi-core speedups", by Bob Friesenhahn
There is no mystery here -- the I/O itself is still serialized, so if it's only 10% of the total runtime, say, even perfectly linear thread scaling and an unlimited number of cores without any lock contention will experience a maximum of 10x improvement. Amdahl's Law and all that. That effect alone adequately explains my lack of thread scalability beyond 8 or so cores, for this application.
The I/O is not necessarily serialized. When using mmap, pread/pwrite, or asynchronous I/O, the I/O can also benefit from multi-threading given that the underlying storage is a multi-disk array or SSDs using PCIe NVMe (capable of up to 1024 simultaneous I/Os).
Amdahl's Law still applies. :-)
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