This provides useful and not too noisy output at INFO level. We do an
info-level message on every block commit instead of trying to do one
message every N blocks, because this is useful both for initial block
sync as well as continuous state updates on new blocks.
The metrics code becomes much simpler because the current version of the
metrics crate builds its own single-threaded runtime on a dedicated worker
thread, so no dependency on the main Zebra Tokio runtime is required.
This change is mostly mechanical, with the exception of the changes to the
`tower-batch` middleware. This middleware was adapted from `tower::buffer`,
and the `tower::buffer` code was changed to implement its own bounded queue,
because Tokio 0.3 removed the `mpsc::Sender::poll_send` method. See
ddc64e8d4d
for more context on the Tower changes. To match Tower as closely as possible
in order to be able to upstream `tower-batch`, those changes are copied from
`tower::Buffer` to `tower-batch`.
## Motivation
Prior to this PR we've been using `sled` as our database for storing persistent chain data on the disk between boots. We picked sled over rocksdb to minimize our c++ dependencies despite it being a less mature codebase. The theory was if it worked well enough we'd prefer to have a pure rust codebase, but if we ever ran into problems we knew we could easily swap it out with rocksdb.
Well, we ran into problems. Sled's memory usage was particularly high, and it seemed to be leaking memory. On top of all that, the performance for writes was pretty poor, causing us to become bottle-necked on sled instead of the network.
## Solution
This PR replaces `sled` with `rocksdb`. We've seen a 10x improvement in memory usage out of the box, no more leaking, and much better write performance. With this change writing chain data to disk is no longer a limiting factor in how quickly we can sync the chain.
The code in this pull request has:
- [x] Documentation Comments
- [x] Unit Tests and Property Tests
## Review
@hdevalence
This helps prevent overloading the network with too many concurrent
block requests. On a fast network, we're likely to still have enough
room to saturate our bandwidth. In the worst case, with 2MB blocks,
downloading 50 blocks concurrently is 100MB of queued downloads. If we
need to download this in 20 seconds to avoid peer connection timeouts,
the implied worst-case minimum speed is 5MB/s. In practice, this
minimum speed will likely be much lower.
This reverts commit 656bd24ba7.
The Hedge middleware keeps a pair of histograms, writing into one in the
current time interval and reading from the previous time interval's
data. This means that the reverted change resulted in doubling all
block downloads until after at least the second measurement interval
(which means that the time measurements are also incorrect, as they're
operating under double the network load...)
* Use the default memory limit in the acceptance tests
PR #1233 changed the default `memory_cache_bytes`, but left the
acceptance tests with their old value.
Sets the default value to the previous lookahead limit. My testing on
mainnet suggested that the newly lower value (changed when the
checkpoint frequency was decreased) is low enough to cause stalls, even
when using hedged requests.
Remove the minimum data points from the syncer hedge configuragtion.
When there are no data points, hedge sends the second request
immediately.
Where there are less than 1/(1-latency_percentile) data points (20),
hedge delays the second request by the highest recent download time.
This change should improve genesis and post-restart sync latency.
* Run large checkpoint sync tests in CI
* Improve test child output match error context
* Add a debug_stop_at_height config
* Use stop at height in acceptance tests
And add some restart acceptance tests, to make sure the stop at
height feature works correctly.
We should error if we notice that we're attempting to download the same
blocks multiple times, because that indicates that peers reported bad
information to us, or we got confused trying to interpret their
responses.
The original sync algorithm split the sync process into two phases, one
that obtained prospective chain tips, and another that attempted to
extend those chain tips as far as possible until encountering an error
(at which point the prospective state is discarded and the process
restarts).
Because a previous implementation of this algorithm didn't properly
enforce linkage between segments of the chain while extending tips,
sometimes it would get confused and fail to discard responses that did
not extend a tip. To mitigate this, a check against the state was
added. However, this check can cause stalls while checkpointing,
because when a checkpoint is reached we may suddenly need to commit
thousands of blocks to the state. Because the sync algorithm now has a
a `CheckedTip` structure that ensures that a new segment of hashes
actually extends an existing one, we don't need to check against the
state while extending a tip, because we don't get confused while
interpreting responses.
This change results in significantly smoother progress on mainnet.
There's no reason to return a pre-Buffer'd service (there's no need for
internal access to the state service, as in zebra-network), but wrapping
it internally removes control of the buffer size from the caller.
The timeout behavior in zebra-network is an implementation detail, not a
feature of the public API. So it shouldn't be mentioned in the doc
comments -- if we want timeout behavior, we have to layer it ourselves.
Using the cancel_handles, we can deduplicate requests. This is
important to do, because otherwise when we insert the second cancel
handle, we'd drop the first one, cancelling an existing task for no
reason.
The hedge middleware implements hedged requests, as described in _The
Tail At Scale_. The idea is that we auto-tune our retry logic according
to the actual network conditions, pre-emptively retrying requests that
exceed some latency percentile. This would hopefully solve the problem
where our timeouts are too long on mainnet and too slow on testnet.
Try to use the better cancellation logic to revert to previous sync
algorithm. As designed, the sync algorithm is supposed to proceed by
downloading state prospectively and handle errors by flushing the
pipeline and starting over. This hasn't worked well, because we didn't
previously cancel tasks properly. Now that we can, try to use something
in the spirit of the original sync algorithm.
This makes two changes relative to the existing download code:
1. It uses a oneshot to attempt to cancel the download task after it
has started;
2. It encapsulates the download creation and cancellation logic into a
Downloads struct.
Deirdre Connolly
Jane Lusby
dependabot[bot]
Henry de Valence
teor
Jane Lusby
Alfredo Garcia