We do a lot of work at FP Complete in the realm of multi-machine
computing for our customers. One of the things we need to do
efficiently to get good speedups from adding more machines is
optimizing serialization. Since our multi-machine work often
revolves around scientific and mathematical computing, we primarily
need to make sure that there is efficient serialization of
primitive data types (like Int64
and
Double
), strict/unpacked product types (e.g.
data Foo = Foo !Int64 !Double
), and vectors of any of
these.
For the impatient: benchmark results and repo
Not too long ago Francesco
discovered and reported some issues with the binary package's
serialization of Double
s. In response to this, we
internally switched over to using the cereal package instead, which
resulted in
a few more performance related PRs. However, when the FP
Complete team discussed serialization, and particularly the needs
that our multi-machine computing and other client projects require,
we realized that we may have some opportunities to do better.
Both the binary and cereal packages are very featureful, and with these features come some overhead. We actually have much simpler requirements for our standard serialization cases, e.g.:
Double
s
will be one Int64
(to hold the length of the vector)
plus 50 * sizeOf (undefined :: Double)
, or in other
words: 51 * 8
. By leveraging this, we can possibly do
a more efficient serialization step by allocating a buffer of
exactly the correct sizeWith these ideas in mind, I've spent the past few days putting together a variety of different implementations of both encoding and decoding, and believe that for the limited cases I just described, we can get significantly better serialization performance.
The benchmarks I've used for this experiment focus on a fairly
simple encoding and decoding test for a boxed vector of
SomeData
values. The SomeData
type has a
single data constructor with strict fields of Int64
,
Word8
, and Double
. This is a fairly good
representation of the kind of data that we deal with regularly.
Serializing more complex datatypes, or more variable-length types
like a ByteString
, are certainly supported by all of
the schemes I've described here, but the performance impact of that
has not been tested (since it's not the main focus of our
work).
There are three different binary representations of the data used in this benchmark suite:
Simple little-endian (host-endian) serialization. In this case,
the values are serialized in exactly the same format as they are
stored in memory. (This assumes of course a little-endian
architecture, which is what I did all testing on and our target
architecture in production in almost all cases.)
Vector
s are serialized by storing the length of the
Vector
as an Int64
followed by each
successive element from the Vector
. This format is
used by the following functions:
encodeSimpleRaw
encodeSimplePoke
encodeSimplePokeMonad
encodeSimplePokeRef
encodeSimplePokeRefMonad
encodeBuilderLE
decodeSimplePeek
decodeSimplePeekEx
decodeRawLE
The big-endian format of the above. This format is used by the following functions:
encodeBuilderBE
encodeCereal
decodeRawBE
decodeCereal
The binary format, used exclusively by the binary package. The
special magic in this format is that Double
s are
encoded as a pair of Integer
and Int
.
This is the original performance bug that Francesco found and
reported on Reddit, and which kicked off this whole analysis.
Let's start with the benchmark results, and then do some more analysis:
Unsurprisingly, binary
performed the worse by far,
mainly due to its Double
serialization. Similarly, the
big endian approaches were all slower than the little endian
approaches, though the cereal package performed significantly worse
than the raw bytestring-builder encoding and direct
ByteString/bit-shifting decoding.
Since the little endian encoding wins as far as representation, we'll spend the rest of this approach analyzing the different trade-offs in encoding and decoding, based on the 6 encoding and 3 decoding functions implemented and tested here.
When both encoding and decoding, we need to keep track of the current offset being written to/read from within the buffer. In the case of decoding, we also need to have some way to fail out if the data parsed is not what we expect, or there are insufficient bytes to consume. I tested two approaches for this:
A continuation-based approach, where each computation is passed
the next computation to perform. That next computation takes a
parameter of the updated offset, and in the case of decoding
returns a Maybe
value to allow for failure. This
technique is used by the functions:
encodeSimpleRaw
encodeSimplePoke
encodeSimplePokeMonad
decodeSimplePeek
decodeRawLE
A mutable reference approach. In this case, instead of each
function needing to take an updated offset value, the value is
stored in a mutable reference. This allows the environment
available to each function to be identical (i.e., a pure
ReaderT
setup), which GHC is good at optimizing. On
the other hand, GHC also tends to do much better at optimizing code
without mutable references in it. To deal with failed decoding in
this setup, we use runtime exceptions. This technique is used by
the functions:
encodeSimplePokeRef
encodeSimplePokeRefMonad
decodeSimplePeekEx
As you can see from the results, the continuation based approach gave a slight performance advantage. While this is what I would have expected, the difference between the two was less than I had expected. Additionally, in some earlier testing before I'd added more optimization, the reference based implementation actually outperformed the continuation based implementation. The details of what's going on at the core level would be an interesting follow-up analysis.
If you dive into the code, you may be a bit surprised at how the
offset reference is represented. Instead of a more standard
IORef
, we have:
newtype OffsetRef = OffsetRef (Data.Vector.Unboxed.Mutable.MVector RealWorld Offset)
The idea here is to take advantage of the more efficient unboxed
representation and byte array operations provided by the vector
package and GHC. This also avoids a lot of garbage collection
overhead used by the boxed nature of IORef
. This is
the same technique leveraged by the mutable-containers
package. Also, check out
the Stack Overflow question on the topic.
When it comes to storing primitive datatypes in a pointer, it's unsurprising to see the Storable type class. This class's peek and poke operations are implemented under the surface with GHC primops. We get away with this because, in our implementation, we are always using host byte order. By contrast, the cereal, binary, and bytestring-builder packages need to do more direct bit-shifting, such as:
word64be :: ByteString -> Word64 word64be = \s -> (fromIntegral (s `SU.unsafeIndex` 0) `shiftL` 56) .|. (fromIntegral (s `SU.unsafeIndex` 1) `shiftL` 48) .|. (fromIntegral (s `SU.unsafeIndex` 2) `shiftL` 40) .|. (fromIntegral (s `SU.unsafeIndex` 3) `shiftL` 32) .|. (fromIntegral (s `SU.unsafeIndex` 4) `shiftL` 24) .|. (fromIntegral (s `SU.unsafeIndex` 5) `shiftL` 16) .|. (fromIntegral (s `SU.unsafeIndex` 6) `shiftL` 8) .|. (fromIntegral (s `SU.unsafeIndex` 7) )
Leveraging Storable
whenever possible simplifies
our codebase and makes it more efficient. In fact, the
Simple
typeclass - used for most of our
implementations here - provides default implementations of all
functions in terms of Storable
, so that the instances
for primitive types just look like:
instance Simple Int64 instance Simple Word8 instance Simple Double
You may be wondering: if Storable
is so great, why
not just base a serialization library on it directly? There's one
downside: it assumes that every datatype has a fixed serialized
width. While this works great for Int
s and the like,
it fails with a Vector
, where we need to know - at
runtime - the actual length of the Vector
to determine
its serialized size.
A naive implementation of this would look like:
simpleSize :: a -> Int
However, this signature implies that every type's size may
change at runtime. This would require that, when serializing a
Vector
, we always inspect every single value, which
introduces significant CPU overhead. For example, given that the
representation of a Vector
is an 8-byte integer length
plus the sizes of all elements, this would look like:
simpleSize :: Vector a -> Int simpleSize v = 8 + V.sum (V.map simpleSize v)
But in the case of statically-known sizes, we'd like to replace
that sum
with a simple multiplication. To make this
work, we have a slightly smarter type signature for the size
function:
simpleSize :: Either Int (a -> Int)
If this function returns Left
, there's no need to
inspect individual values. For Right
, we're stuck with
checking each thing. Putting this together, here's our
implementation for Vector
s.
simpleSize :: Either Int (Vector a -> Int) simpleSize = Right $ \v -> case simpleSize of Left s -> s * V.length v + 8 Right f -> V.sum (V.map f v) + 8
Notice how, for a Vector Int64
, we'd be able to
follow the Left
branch and avoid the costly
sum
. This ends up giving us really cheap knowledge of
the total buffer size needed, which we take advantage of when
encoding, e.g.:
encodeSimpleRaw :: Simple a => a -> ByteString encodeSimpleRaw x = unsafeCreate (either id ($ x) simpleSize) (\p -> simpleRawPoke p 0 x)
Ever since blaze-html
famously provided a broken
Monad
interface in the name of performance, there's
been a concern (at least by me) that providing a proper
Monad
instance instead of just a Monoid
instance could slow things down. Therefore, this benchmark included
encoding functions that are both monadic
(encodeSimplePokeMonad
and
encodeSimplePokeRefMonad
) and monoidal
(encodeSimplePoke
and
encodeSimplePokeRef
). To my surprise, they performed
nearly identically.
The monadic interface though has many advantages over a monoidal one:
Monoid
instance for any
Monad
, so you actually get both interfaces for
freedo
notation if desiredmapM_
vs
foldMap
).foldMap
from Vector
. I'm guessing this is
due to a missing INLINE
in the default implementation
of foldMap
in Data.Foldable
, but I
haven't had a chance to investigate yet. Again, since monadic
combinators seem to be more well used, odds are that they will also
be more well optimized.In addition to the nice newtype
-wrapped monads and
monoids, I implemented a raw version of both encoding and decoding,
just to see if the abstraction added an overhead. Fortunately, this
was not the case, which lets us stick with relatively simple
high-level code such as:
simplePeek = SomeData <$> simplePeek <*> simplePeek <*> simplePeek
instead of:
readInt64 bs $ \bsX x -> readWord8 bsX $ \bsY y -> readDouble bsY $ \bsZ z -> do MV.unsafeWrite mv idx (SomeData x y z) loop (idx + 1) bsZ
If you look at
the implementation of the Binary
instance for
Vector
, you'll see that it needs to resort to
unsafePerformIO
. This is because, in order to
efficiently create a Vector
, it needs to perform
mutations, but the Get
monad from binary does not
provide any ST
or IO
like behavior for
mutating a buffer.
Since we're designing a new decoding interface from scratch, and
have Vector
as a first-class use case, I decided to
bake in ST
-like semantics to allow directly playing
around with mutable vectors. As a result, the type of
Peek
has an extra state token parameter, just like
ST
:
newtype Peek s a
Also, there is a PrimMonad
instance for
Peek
, which allows for the mutable vector operations
to occur within the monad without any lifting. To see how this
works, take a look at the implementation of simplePeek
for Vector
s:
simplePeek :: Simple a => Peek (Vector a) simplePeek = do len :: Int64 <- simplePeek let len' = fromIntegral len mv <- MV.new len' let loop i | i >= len' = V.unsafeFreeze mv | otherwise = do x <- simplePeek MV.unsafeWrite mv i x loop $! i + 1 loop 0
All of this also applies to the PeekEx
type.
The criterion link I shared above has the results of all of these benchmarks, but for the lazy, here's the result graph again:
As you can see: the little-endian encoding regularly outperformed the other two formats by a significant margin. This isn't surprising, given that there is less CPU work that needs to be done, and that we are able to leverage primops from GHC to do most of the work.
Similarly, while bytestring-builder is a very highly tuned
library, for a narrow use case like ours where the resulting buffer
size is easily computed in advance, constructing
ByteString
s manually gives a significant performance
boost.
To my surprise, the mutable reference/exception based approach to peeking and poking was fairly competitive with the continuation-based approach. Nonetheless, overall the continuation-based approach outperforms it, and is more elegant an approach.
While the monadic and monoidal interfaces for poking/encoding
give similar results, the performance of the monoidal interface can
be unpredictable if used incorrectly. Since monadic combinators are
more commonly optimized and more ubiquitous, it's safer to expose
the monadic interface. (And, for what it's worth, we can always
provide a Monoid
instance for any
Monad
.)
And thankfully, the abstractions we use to make our encoding and decoding easy to work with and nicely composable do not introduce a slowdown versus the low-level implementation. So no need to force people to drop down to manual continuation passing.
Based on this analysis, it seems like we have a clear set of choices for a new serialization library. This blog post already clarifies the design goals of such a library, and its limitations (e.g., no streaming decoding). Such a library is something we'll almost certainly want to leverage for our multi-machine computation work at FP Complete.
I'm interested on feedback from others on this topic, including ways to better optimize the code, alternative strategies, and additional use cases you might have for such a library.
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