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Related: anyone know of a good video or diagram for helping CS students get an intuition regarding the interplay of bandwidth and latency? Including how saturat

Related: anyone know of a good video or diagram for helping CS students get an intuition regarding the interplay of bandwidth and latency? Including how saturating bandwidth increases latency by causing queueing bottlenecks? I'm looking for something a bit more visual and dynamic than the old "station wagon full of tapes going down the highway" imagery. [EDIT: Just for clarification, I feel like I already have a pretty good grasp on these concepts. I'm looking for good ways to help others at the ~ undergrad level.]

wtallis 5y ago
I do some rather coarse measurements of random read throughput vs latency as part of my SSD reviews. See eg. the bottom of https://www.anandtech.com/show/16636/the-inland-performance-plus-2tb-ssd-review-phisons-e18-nvme-controller-tested/3 https://www.anandtech.com/show/16636/the-inland-performance-... Those graphs cut off the essentially vertical latency spike that results from enqueuing requests faster than the sustained rate at which the drive can serve them. For a different view in terms of queue depth rather than throughput, there are some relevant graphs from an older review that predates io_uring: https://www.anandtech.com/show/11930/intel-optane-ssd-dc-p4800x-750gb-handson-review/5 https://www.anandtech.com/show/11930/intel-optane-ssd-dc-p48... Generally speaking, latency starts increasing long before you've reached a drive's throughput limit. Some of this is inevitable, because you have a relatively small number of channels (eg. 8) and dies to access in parallel. Once you're up to the throughput range where you have dozens of requests in flight at a time, you'll have constant collisions where multiple requests want to read from the same plane/die/channel at once, and some of those requests have to be delayed. But that's mostly about contention and link utilization between the SSD controller and the NAND flash itself. The PCIe link is pretty good about handling transactions with consistently low latency even when on average it's mostly busy.
bombcar 5y ago
Here's a post on relative latencies that may be useful: https://danluu.com/infinite-disk/ https://danluu.com/infinite-disk/ There was another post I saw recently comparing the increase in disk SIZE over the last 30 years vs the increase in disk SPEED vs LATENCY (so size in GB, speed in GB/s, latency in IOPS) - and how size increases far outstripped speed which outstripped latency, though all had improved. Found it! The key is IOPS/GB as a metric. https://brooker.co.za/blog/2021/03/25/latency-bandwidth.html https://brooker.co.za/blog/2021/03/25/latency-bandwidth.html
uyt 5y ago
Are you referring to Little's Law?
louwrentius 5y ago
Maybe this doesn’t answer your question exactly but I addressed this topic in two blogpost, maybe it helps. https://louwrentius.com/understanding-storage-performance-iops-and-latency.html https://louwrentius.com/understanding-storage-performance-io... https://louwrentius.com/understanding-iops-latency-and-storage-performance.html https://louwrentius.com/understanding-iops-latency-and-stora...

wtallis 5y ago

I do some rather coarse measurements of random read throughput vs latency as part of my SSD reviews. See eg. the bottom of https://www.anandtech.com/show/16636/the-inland-performance-plus-2tb-ssd-review-phisons-e18-nvme-controller-tested/3 https://www.anandtech.com/show/16636/the-inland-performance-... Those graphs cut off the essentially vertical latency spike that results from enqueuing requests faster than the sustained rate at which the drive can serve them. For a different view in terms of queue depth rather than throughput, there are some relevant graphs from an older review that predates io_uring: https://www.anandtech.com/show/11930/intel-optane-ssd-dc-p4800x-750gb-handson-review/5 https://www.anandtech.com/show/11930/intel-optane-ssd-dc-p48... Generally speaking, latency starts increasing long before you've reached a drive's throughput limit. Some of this is inevitable, because you have a relatively small number of channels (eg. 8) and dies to access in parallel. Once you're up to the throughput range where you have dozens of requests in flight at a time, you'll have constant collisions where multiple requests want to read from the same plane/die/channel at once, and some of those requests have to be delayed. But that's mostly about contention and link utilization between the SSD controller and the NAND flash itself. The PCIe link is pretty good about handling transactions with consistently low latency even when on average it's mostly busy.

bombcar 5y ago

Here's a post on relative latencies that may be useful: https://danluu.com/infinite-disk/ https://danluu.com/infinite-disk/ There was another post I saw recently comparing the increase in disk SIZE over the last 30 years vs the increase in disk SPEED vs LATENCY (so size in GB, speed in GB/s, latency in IOPS) - and how size increases far outstripped speed which outstripped latency, though all had improved. Found it! The key is IOPS/GB as a metric. https://brooker.co.za/blog/2021/03/25/latency-bandwidth.html https://brooker.co.za/blog/2021/03/25/latency-bandwidth.html

uyt 5y ago

Are you referring to Little's Law?

louwrentius 5y ago

Maybe this doesn’t answer your question exactly but I addressed this topic in two blogpost, maybe it helps. https://louwrentius.com/understanding-storage-performance-iops-and-latency.html https://louwrentius.com/understanding-storage-performance-io... https://louwrentius.com/understanding-iops-latency-and-storage-performance.html https://louwrentius.com/understanding-iops-latency-and-stora...