Revert "Background merging of deserialized scripts"
This reverts commit e895b7af. Reason for revert: TSAN failures: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20TSAN%20-%20stress-incremental-marking/8468/overview Original change's description: > Background merging of deserialized scripts > > Recently, https://crrev.com/c/v8/v8/+/3681880 added new API functions > with which an embedder could request that V8 merge newly deserialized > script data into an existing Script from the Isolate's compilation > cache. This change implements those new functions. This functionality is > still disabled by default due to the flag > merge_background_deserialized_script_with_compilation_cache. > > The goal of this new functionality is to reduce memory usage when > multiple frames load the same script with a long delay between (long > enough for the script to have been evicted from Blink's in-memory cache > and for the top-level SharedFunctionInfo to be flushed). In that case, > there are two Script objects for the same script: one which was found in > the Isolate compilation cache (the "old" script), and one which was > recently deserialized (the "new" script). The new script's object graph > is essentially standalone: it may point to internalized strings and > readonly objects such as the empty feedback metadata, but otherwise > it is unconnected to the rest of the heap. The merging logic takes any > useful data from the new script's object graph and attaches it into the > old script's object graph, so that the new Script object and any other > duplicated objects can be discarded. More specifically: > > 1. If the new Script has a SharedFunctionInfo for a particular function > literal, and the old Script does not, then the old Script is updated > to refer to the new SharedFunctionInfo. > 2. If the new Script has a compiled SharedFunctionInfo for a particular > function literal, and the old Script has an uncompiled > SharedFunctionInfo, then the old SharedFunctionInfo is updated to > point to the function_data and feedback_metadata from the new > SharedFunctionInfo. > 3. If any used object from the new object graph points to a > SharedFunctionInfo, where the old object graph contains a matching > SharedFunctionInfo for the same function literal, then that pointer > is updated to point to the old SharedFunctionInfo. > > The document at [0] includes diagrams showing an example merge on a very > small script. > > Steps 1 and 2 above are pretty simple, but step 3 requires walking a > possibly large set of objects, so this new API lets the embedder run > step 3 from a background thread. Steps 1 and 2 are performed later, on > the main thread. > > The next important question is: in what ways can the old script's object > graph be modified during the background execution of step 3, or during > the time after step 3 but before steps 1 and 2? > > A. SharedFunctionInfos can go from compiled to uncompiled due to > flushing. This is okay; the worst outcome is that the function would > need to be compiled again later. Such a risk is already present, > since V8 doesn't keep IsCompiledScopes for every compiled function in > a background-deserialized script. > B. SharedFunctionInfos can go from uncompiled to compiled due to lazy > compilation. This is also okay; the merge completion logic on the > main thread will just keep this lazily compiled data rather than > inserting compiled data from the newly deserialized object graph. > C. SharedFunctionInfos can be cleared from the Script's weak array if > they are no longer referenced. This is mostly okay, because any > SharedFunctionInfo that is needed by the background merge is strongly > referenced and therefore can't be cleared. The only problem arises if > the top-level SharedFunctionInfo gets cleared, so the merge task must > deliberately keep a reference to that one. > D. SharedFunctionInfos can be created if they are needed due to lazy > compilation of a parent function. This change is somewhat troublesome > because it invalidates the background thread's work and requires a > re-traversal on the main thread to update any pointers that should > point to this lazily compiled SharedFunctionInfo. > > At a high level, this change implements three previously unimplemented > functions in BackgroundDeserializeTask (in compiler.cc) and updates one: > > - BackgroundDeserializeTask::SourceTextAvailable, run on the main > thread, checks whether there is a matching Script in the Isolate > compilation cache which doesn't already have a top-level > SharedFunctionInfo. If so, it saves that Script in a persistent > handle. > - BackgroundDeserializeTask::ShouldMergeWithExistingScript checks > whether the persistent handle from the first step exists (a fast > operation which can be called from any thread). > - BackgroundDeserializeTask::MergeWithExistingScript, run on a > background thread, performs step 3 of the merge described above and > generates lists of persistent data describing how the main thread can > complete the merge. > - BackgroundDeserializeTask::Finish is updated to perform the merge > steps 1 and 2 listed above, as well as a possible re-traversal of the > graph if required due to newly created SharedFunctionInfos in the old > Script. > > The merge logic has nothing to do with deserialization, and indeed I > hope to reuse it for background compilation tasks as well, so it is all > contained within a new class BackgroundMergeTask (in compiler.h,cc). It > uses a second class, ForwardPointersVisitor (in compiler.cc) to perform > the object visitation that updates pointers to SharedFunctionInfos. > > [0] https://docs.google.com/document/d/1UksB5Vm7TT1-f3S9W1dK_rP9jKn_ly0WVm_UDPpWuBw/edit > > Bug: v8:12808 > Change-Id: Id405869e9d5b106ca7afd9c4b08cb5813e6852c6 > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3739232 > Reviewed-by: Leszek Swirski <leszeks@chromium.org> > Commit-Queue: Seth Brenith <seth.brenith@microsoft.com> > Cr-Commit-Position: refs/heads/main@{#81941} Bug: v8:12808 Change-Id: I82a080e6287828445293cb6b4b94a5e8f15eb8f3 No-Presubmit: true No-Tree-Checks: true No-Try: true Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3787213 Auto-Submit: Deepti Gandluri <gdeepti@chromium.org> Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com> Owners-Override: Deepti Gandluri <gdeepti@chromium.org> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com> Cr-Commit-Position: refs/heads/main@{#81943}
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