With the introduction of layered frames, each database lookup may result
in hundreds of table lookups as the frame stack is traversed.
This change restores performance by introducing snapshots to limit the
lookup depth at the expense of slightly increased memory usage.
The snapshot contains the cumulative changes of all ancestors and itself
allowing the lookup recursion to stop whenever it is encountered.
The number of snapshots to keep in memory is a tradeoff between lookup
performance and memory usage - this change starts with a simple strategy
of keeping snapshots for head frames (approximately). T
he snapshot is created during checkpointing, ie after block validation,
to make sure that it's cheap to start verifying blocks - parent
snapshots are moved to the descendant as part of checkpointing which
effectively means that head frames hold snapshots in most cases.
The outcome of this tradeoff is that applying a block to a known head is
fast while creating a new branch of history remains expensive.
Another consequence is that when persisting changes to disk, we must
re-traverse the stack of changes to build a cumulative set of changes to
be persisted.
A future strategy might be to keep additional "keyframes" along the way,
ie one per epoch for example - this would bound the "branch creation"
cost to a constant factor, but memory overhead should first be
considered.
Another strategy might be to avoid keeping snapshots for non-canonical
branches, specially when they become older and thus less likely to be
branched from.
* `level` is updated to work like a temporary serial number to maintain
its relative position in the sorting order as frames are persisted
* a `snapshot` is added to some TxFrame instances - the snapshot
collects all ancestor changes up to and including the given frame.
`level` is used as a marker to prune the snapshot of changes that have
been persisted already.
* stack traversals for the purpose of lookup stop when they encounter a
snapshot - this bounds the lookup depth to the first encountered
snapshot
After this PR, sync performance lands at about 2-3 blocks per second
(~10x improvement) - this is quite reasonable when comparing with block
import which skips the expensive state root verification and thus
achieves ~20 blk/s on the same hardware. Additional work to bring live
syncing performance in line with disk-based block import would focus on
reducing state root verification cost.
* Simplify txFrame protocol, improve persist performance
To prepare forked-layers for further surgery to avoid the nesting tax,
the commit/rollback style of interacting must first be adjusted, since
it does not provide a point in time where the frame is "done" and goes
from being actively written to, to simply waiting to be persisted or
discarded.
A collateral benefit of this change is that the scheme removes some
complexity from the process by moving the "last saved block number" into
txframe along with the actual state changes thus reducing the risk that
they go "out of sync" and removing the "commit" consolidation
responsibility from ForkedChain.
* commit/rollback become checkpoint/dispose - since these are pure
in-memory constructs, there's less error handling and there's no real
"rollback" involved - dispose better implies that the instance cannot be
used and we can more aggressively clear the memory it uses
* simplified block number handling that moves to become part of txFrame
just like the data that the block number references
* avoid reparenting step by replacing the base instead of keeping a
singleton instance
* persist builds the set of changes from the bottom which helps avoid
moving changes in the top layers through each ancestor level of the
frame stack
* when using an in-memory database in tests, allow the instance to be
passed around to enable testing persist and reload logic
