Increase tags count in TagList for no allocation from 8 to a higher value

[dpk83]

Feature Request

TagList struct provides a way to avoid allocating memory when capturing metrics with upto 8 tags. This really helps achieve optimized paths for metric collection. While 8 tags works for close to 60-70% cases we still have quite a few cases where we the metrics are running above 8 tags.

The ask is to increase the number of tags for allocation free path from 8 to 12. We are running pretty close to 10 for some of our common metrics and we think that 12 should be a good tradeoff where it provides us a little wiggle-room while keeping the number still manageable and help most of the metric collections be allocation free.

[dpk83]

@noahfalk @tarekgh

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Issue Details

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Feature Request

TagList struct provides a way to avoid allocating memory when capturing metrics with upto 8 tags. This really helps achieve optimized paths for metric collection. While 8 tags works for close to 60-70% cases we still have quite a few cases where we the metrics are running above 8 tags.

The ask is to increase the number of tags for allocation free path from 8 to 12. We are running pretty close to 10 for some of our common metrics and we think that 12 should be a good tradeoff where it provides us a little wiggle-room while keeping the number still manageable and help most of the metric collections be allocation free.

Author:	dpk83
Assignees:	-
Labels:	untriaged, area-System.Diagnostics.Activity
Milestone:	8.0.0

[stephentoub]

Is the intent of TagList that it's invariably constructed to live on the stack and rarely if ever as part of an object that's on the heap?

[tarekgh]

Is the intent of TagList that it's invariably constructed to live on the stack and rarely if ever as part of an object that's on the heap?

It is intended to be used on the stack mostly to pass a variable list of tags to our APIs without allocation.

[stephentoub]

It is intended to be used on the stack mostly to pass a variable list of tags to our APIs without allocation.

Ok. It'd be worth measuring the impact the proposed change has. It's going to increase the amount of zero'ing required in the containing method call for all uses of TagList by another 64 bytes. If the majority of uses don't benefit, it's possible it could be a net negative. If it helps avoid a lot of allocation, it could of course be well worth it.

Just pointing out it's not "free".

[tarekgh]

When we introduced the TagList, we chose 8 as a guess for OTel common scenarios. But, we were watching if it is enough for mainstream cases. According to @dpk83 he said We are running pretty close to 10 for some of our common metrics. which suggests 8 is not good enough in the common cases. And yes, we understand it has some cost too for the initialization, but it is going to save a lot of allocation. The telemetry APIs which can use TagList usually is called more frequently.