This saves a whole bunch of memory. Cutting `Just Cause 2`'s title
screen from 1132MB anonymous FEX memory down to 438MB. 629MB in L2
alone.
L2 is primarily a means to reduce overhead in map queries, so it's all
about performance. But because it consumes a lot of people it's kind of
hard.
One idea is that the L2 lookups can be moved to shared data structures,
since we already pull the shared lock when doing an L2 lookup this is
already halfway there.
Side note, we're using unique locks even with read-only code paths
which we can't use the shared lock because this terrible recursive
mutex!
Instead of outright changing L2 behaviour and potentially wrecking
havoc, add a config option for now so testing can happen over time.
before:
```
Total FEX Anon memory resident: 1132 mB
JIT resident: 60 mB
OpDispatcher resident: 97 mB
Frontend resident: 37 mB
CPUBackend resident: 500 kB
Lookup cache resident: 629 mB
Lookup L1 cache resident: 108 mB
ThreadStates resident: 436 kB
```
after:
```
Total FEX Anon memory resident: 438 mB
JIT resident: 62 mB
OpDispatcher resident: 56 mB
Frontend resident: 22 mB
CPUBackend resident: 496 kB
Lookup cache resident: 0 (null)
Lookup L1 cache resident: 109 mB
ThreadStates resident: 436 kB
```
Every time I see this recursive mutex I glare at it. Remove the last one
so that we no longer need to deal with it.
The only reason why this recursive mutex still existed today was because
it is fairly intertwined with the ContextImpl and tracing it all was a
pain.
Peel back the layers and follow the idiom to have ContextImpl pull the
write mutex when requiredand pass it through by reference to ensure it stays alive.
This allows us to entirely give rid of the recursive nature of the
mutex, which means that `FindBlock` can eventually be switched over to a
read-lock to improve multiple threads reading the caches at the same
time.
I didn't do that exercise since that can be followed up in a subsequent
PR.
Prevents invalidations being missed under the following circumstances:
Thread A JITs block A into the global codebuffer, adding the guest to host
mapping to its CodePages, thread A is then killed.
Thread B then performs SMC on block A. An exception will be triggered but
as CodePages was stored per-thread, and thread A is now killed when all
threads are iterated over by the frontend to perform invalidations it
will be missed.
The accumulator is introduced to handle the case where multiple threads
have the same code entry in their local caches but share the same codebuffer.
Consider a thread C in the above example that also has block A in its cache,
without an accumulator, when invalidating thread B the entrypoint of A is erased
from the shared guest to host map. So when C is invalidated, the local cache entry
for A is not removed since it was removed from CodePages when invalidating B.
This is changes the interface of CodeBuffer to that of a partially persistent
data structure based on reference counting:
- Exactly one CodeBuffer is now designated as "active", which means data can
be *appended* to it
- Lossy modifications to the active CodeBuffer will not invalidate any data
in use by other threads, which enables save sharing across threads
- Instead, such lossy modifications trigger a new "version" of the data in
the modifying thread. Old versions of the CodeBuffer persist as read-only
data for use by the other threads.
- The other threads can update their version of the CodeBuffer. This will
decrease the reference count and eventually trigger deallocation of the
old version
This data isn't really a cache, since the JIT is directly responsible of
writing its contents. Instead it be considered the source to populate the
L1/L2 caches from.
Furthermore, splitting off this data allows it to be shared across threads
in the future without affecting L1/L2 caches.
Rather than checking the actual EC bitmap in the dispatcher (~6 instrs), this
indirection through the code cache allows just 1 instr for the hot path
of calling repeated EC code/x64 code.
The delinker step of the JIT was using std::function with capture
lambdas that required memory allocation when unnecessary.
Because the compiler can't see through our std::function usage it could
never decompose these by itself.
By passing the Thread's frame and record to the function as arguments
then we can have the signature be a raw function pointer.
This fixes an area of concern from:
https://github.com/FEX-Emu/FEX/blob/main/docs/ProgrammingConcerns.md#stdfunction-and-lambdas
It is not an external component, and it makes paths needlessly long.
Ryan seemed amenable to this when we discussed on IRC earlier.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Ryan Houdek