[FEATURE] Allow defining shared memory size at runtime

[Officeyutong]

Currently the size of shared memory is hard coded in bpftime_shm_internal.hpp and is set to 20MB

This might be too small for some cases. Allow setting that size at runtime using an environment variable

[NobinPegasus]

Should the size be passed as flag?
or we should use environment variable and set it's value using export before invoking the tool?

[Officeyutong]

Should the size be passed as flag?

or we should use environment variable and set it's value using export before invoking the tool?

Use flags to set environment variables

[NobinPegasus]

Can you provide some more guidelines on which files should I make the changes.
And if I am not wrong, we would be able to invoke bpftime like this if implemented correctly. sudo ~/.bpftime/bpftime SHARED_MEMORY=100
Right?

[Officeyutong]

Can you provide some more guidelines on which files should I make the changes. And if I am not wrong, we would be able to invoke bpftime like this if implemented correctly. sudo ~/.bpftime/bpftime SHARED_MEMORY=100 Right?

• CLI is at https://github.com/eunomia-bpf/bpftime/blob/master/tools/cli/main.cpp
• invocation should be like bpftime --shared-memory-size or other things you like. CLI should set environment variable like BPFTIME_SHARED_MEMORY_SIZE for bpftime-agent. Agent should use the value of BPFTIME_SHARED_MEMORY_SIZE if provided