onepagers for MSBuildServer and RAR caching (#11005)

* onepagers for MSBuildServer and RAR caching

* onepagers update based on reviews.

* Update RAR_caching_onepager.md

* Update MSBuild_Server_onepager.md

* Update RAR_caching_onepager.md

documentation/specs/proposed/MSBuild_Server_onepager.md

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+## MSBuild Server
+
+MSBuild server aims to create a persistent entry node for the MSBuild process
+that we would communicate with via a thin client. We want to get from
+the current state of “spawn a complete process for every CLI invocation”
+to “we have a server process in the background and we only spawn a small
+CLI handler that will tell the server what to build”.
+This project is based on an already existing project: [MSBuild Server](https://github.com/dotnet/msbuild/blob/main/documentation/MSBuild-Server.md). 
+We need to re-enable it and figure out the way forward.
+
+### Goals and Motivation
+
+Currently all the MSBuild processes are persistent, except for the entry
+point process which lives only for the duration of the build. Restarting
+this process with each and every build leads to some overhead due to
+startup costs like jitting. It also leads to a loss of continuity mainly
+due to the absence of caching.
+
+The primary aim of the MSBuild server is to reduce this startup
+overhead.
+
+The secondary aim of this project is to enable us to introduce more
+advanced caching and potentially some other performance optimizations
+further down the line. However these aren’t in the current scope.
+
+### Impact
+
+Small performance improvement in the short term. Enabling further
+optimizations in the long term. (these improvements are for the Dev Kit
+and inner loop CLI scenarios)
+
+Getting closer to the possibility of decoupling from Visual Studio. VS is currently 
+acting as a MSBuild server in some ways - they are a persistent process that  invokes 
+portions of the MSBuild. We ideally want to transition into "VS calls our server 
+instead" relation, to make our behavior consistent for both VS and CLI based
+builds.
+
+### Stakeholders
+
+MSBuild Team. Successful handover means turning on the
+feature, dogfooding it for long enough to ensure we have reasonable
+certainty that nothing breaks and then rolling it out.
+We should cooperate with our close repositories like SDK and roslyn 
+to get them to opt in before we roll the feature out.
+
+### Risks
+
+The project was already attempted once, however it was postponed because
+it surfaced a group of bugs that weren’t previously visible due to the
+processes not being persistent. One such example is Nuget authentication caching,
+which was a non-isue for a non persistent process but became a blocker
+due to the cache not being refreshable in-process.
+Most of those bugs should be solved by now, however we can run into some 
+new ones. Unfortunately the nature of the bugs means that these won't become
+apparent until we start with the dogfooding.
+
+### Cost
+Note that these are mostly my rough guess based on my limited knowledge.
+
+A week to figure out how to turn on the MSBuild Server in a way that
+will enable us to dogfood it properly **plus** some overhead for the
+review loop.
+
+A month of developer time for bugfixes assuming that nothing goes
+terribly wrong.
+
+Some PM time to communicate with appropriate teams to ask them for help
+with dogfooding.
+
+### Plan
+
+- In a first month we should aim to get the MSBuild server dogfooded for
+  our MSBuild repository inner development loop. (Coding + review + setting up)
+
+- Second month we will monitor it and fix anything that crops up.
+
+- After that we start dogfooding internally in our neighbor repositories(SDK, roslyn)
+ for as long as we feel   necessary to ensure everything works as intended. I would 
+ give this period one to three months of monitoring + bugfixing when necessary.

documentation/specs/proposed/RAR_caching_onepager.md

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+## RAR caching
+RAR(Resolving of Assembly references) is an optimization for the step in 
+every build where we need to gather the graph of assembly references and pass
+them to the compiler. This process is highly cacheable as the references
+don’t change all that often. Currently we have some limited caching in
+place however the way nodes are assigned work results in frequent cache
+misses.
+
+### Goals and motivations
+
+1ES team wants to isolate their File I/O related to the RAR caching which is causing
+issues to their debugging efforts. This is mostly due to the fact that MSBuild is pulling
+files from all nodes at once which results in a tangled mess of IO that is hard to debug.
+
+Our motivation is a possible performance gain however we’re fine with
+the change as long as the impact is not negative.
+
+### Impact
+
+The only impact we’re concerned about is the performance. There will be
+a tension between the gains from caching and costs due to the IPC from
+the process that will act as the cache repository. We need to ensure
+that this balance will be a net positive performance wise.
+
+### Stakeholders
+
+1ES team, Tomas Bartonek, Rainer Sigwald
+
+1ES team will provide the initial cache implementation. We will review
+their PRs and do the performance evaluations. Handover will be
+successful if nothing breaks and we meet our performance requirements
+(no regression or better still an improvement).
+
+### Risks
+
+Some time ago Roman Konecny estimated the RAR caching to not be worth it
+performance wise. 1ES team claims to have created an implementation that
+will either improve or not change the performance. We need to validate
+this claim and push back in case we find performance regression.
+Thorough testing will be needed especially to ensure the performance
+is not impacted.
+
+The risk is having to figure out a different way to help 1ES team to
+isolate their File I/Os if the caching hurts the performance. This could
+result in a larger project requiring more involvement on our side.
+
+### Cost
+
+Week for reviewing the provided PR. Additional two weeks for performance
+testing conditional on the Perfstar infrastructure being functional.
+Some communication overhead
+
+## Plan
+
+1ES team creates the PR wih the RAR cache implementation.
+
+We review the PR with a special emphasis on the performance side of
+things.
+Then we merge the changes. There is no expected follow up beyond the
+usual maintenance for our codebase.