When the interval is a proper superset of the range with start equal to
interval start, the range must be considered a superset or it will be
completly ignored (since it neither a subset nor it overlaps)
This patch replaces a lot of circumstances in which containers
were being copied with circumstances in which they are shared,
using copy-on-write semantics.
To achieve this, we emulate somewhat the design of go's
native `append` function. Operations on a container may optionally
yield a new container. A container can be marked "frozen",
after which no operation should ever write to it in any way;
that applies both to the container itself and the backing store
it refers to, if any. So for instance, instead of:
c.arrayToBitmap()
we now write:
c = c.arrayToBitmap()
Operations which need to modify a container in any way
need to be able to return a new container, which is a modified
copy of the previous container. This applies to operations
like add/remove, but also to things like unmapping memory-mapped
storage, or changing a container's type.
Bitmaps do not support the same copy-on-write semantics,
currently, but "copying" a bitmap and sharing the containers
instead of duplicating them is *much* cheaper than copying
the containers.
Bitmaps do support a .Freeze method, which currently copies
the previous bitmap, making a new one with the same container
pointers, and freezes the individual containers. Use this
if you need a writeable copy of a bitmap -- the resulting
bitmap can safely have its set of containers modified, and
bitmap operators that would want to modify the containers
will use copy-on-write for that.
The primary motivation of this is to reduce the cost of the
row cache used by fragments. As a secondary issue, the row cache
is no longer updated on writes -- that update was actually a
race condition waiting to happen. Rather, writes to a row
invalidate the cache entry for that row. The row cache is
created by creating a new bitmap, and freezing the relevant
containers from the fragment's storage. In the case where
nothing is being written, the row cache grows to contain
bitmaps containing all those containers, but never copies
any containers. If nothing's being read, the row cache is
never created, and the containers are in general not getting
frozen. The only circumstance where copies have to happen is
when things are read (and thus stored in the row cache) and
later modified. In that case, each read freezes objects, and
the first write to a container after it's been frozen will
create a new copy.
We drop the enterprise/b btree implementation, because we
don't really need it anymore -- we now provide that
implementation by default in the open source product anyway.
Along with this, there's a lot of other changes which
improve support for nil containers, as a cheaper representation
for empty containers. Operations which we know will provide
an empty container can always short-circuit and just yield
a nil *Container. Similarly, operations which would provide
a full container can return a single shared full container
object (which is frozen). The higher-level (non type-specific)
container ops are now using that logic to short-circuit
operations for empty and full containers. (For instance,
difference of anything minus an empty container is the
original thing, union of anything and empty is the original
thing, and so on.)
The Containers interface adds "Update" and "UpdateEvery"
methods, based in part on the "Put" interface provided
by the underlying btree implementation; Update performs
a possible update in-place of a container for a given
key, bypassing the need to replicate the search for that
key in the container. UpdateEvery loops through all the
containers.
Containers do not strictly guarantee that they won't
return nil `*Container` objects. However, the container
iterators won't return those -- empty containers aren't
interesting. Some tests are updated to reflect this.
Some of the container internals, like N(), or the isArray()
and related functions, accept nil container pointers. Some,
like Thaw(), do not. For the array(), bitmap(), and runs()
methods, roaringparanoia enables an explicit panic on a nil
container explaining the problem, but the intent is that those
should never be called unless you already know you have the
right kind of container, so by default they don't perform
the extra checks. In most cases, this is already covered
because a nil container is empty, and there's no operation
we can perform that requires us to inspect the contents of
an empty container. This is passing a fair amount of testing,
but the testing may not be comprehensive enough.
The overall impact of this is pretty trivial performance-wise.
In our default roaring/ benchmarks, a few things get a few
percent faster, or slower. The advantage is that, with
read-heavy workloads, the row cache no longer eats up incredible
amounts of memory.
For a smallish test case, pilosa's memory usage (RES in top) after
startup was ~2.5GB. Without this patch, simply reading every
row a few times got memory usage to about 9GB, which seemed
reasonably stable. With this patch, memory usage went to about
3GB. This will be less noticeable in mixed read/write loads,
but it should be consistently significantly lower.
In addition to dropping things from the rowCache on modifications,
we also stopped performing a full count on a modified row when
not using a cache of a kind that would use that count, and don't
repopulate the rowCache regardless. We don't want every write
to imply a corresponding read after it.
There's a lot of room for possible future optimizations in
terms of things like in-place operations, and some of the
row/rowSegment code is a little suspicious to me, but I don't
think it should be *worse* in any cases.
So with the switch to a new linter, we get a lot of new warnings,
and the majority of them are harmless probably, but a few might be
real. Variously just use _ to suppress warnings, or report errors.
There's probably things here that deserve better fixes, but we can
always revisit it.
On a 64-bit machine, the slices in a Container consume 72
bytes, and the Container itself is 80. But we only use one
slice at a time! This patch shifts us to keeping a single
slice in the Container, and converting provided slices to
and from that type when we want to update it. (It is not
safe to access the slice through the wrong type.)
We also add some new tests, conditional on a build tag
called `roaringparanoia`. These tests will be optimized
away entirely by the compiler when the tag isn't
present, because the conditionals use a const. These catch
possible errors like trying to access the bitmap slice
of a non-bitmap container.
We also eliminate all direct creation of Container literals,
so we can mess with the internals more. (On reflection
and study, we decided not to go to the fancier design where
references to .n and .typ were also converted to function
calls, which would have allowed packing those attributes
more tightly, because it was a lot more overhead and a lot
of work to keep track of.)
There's some circumstances where we appear to have been
relying on incorrect guesses about the nature of containers.
For instance, in xorBitmapRun, there's logic that makes sense
only if the output's a run container, but it's not, it's a
bitmap container. This creates strange behavior sometimes,
though. Several of these are corrected now.
Add a benchmark to test a specific case where UnionInPlace is
underperforming the naive union operation badly.
Also, the UnionBulk test was reusing a bitmap, meaning that it ended
up doing a lot of unions into a bitmap that already had all the
bits it was supposed to have. This broke a couple of other tests
in unexpected ways.
We also now use UnionInPlace in importRoaring, and test it
in the container combinations tests via a wrapper.
Two changes: First, make SliceDescending set the entire
slice, not all-but-one bits. Second, add tests that are
"striped", so it's writing to 8 parts of the slice
sequentially, rather than just going up or down the whole
thing, because that gives us some cheap indication of
cache-locality impact, which turns out to be possibly
significant.
A transient bug introduced in intersectionCountArrayArray was
not caught by the tests, because it would only manifest when
two containers of different lengths were being compared. Also
improve the testing for intersectArrayArray, even though that
code hasn't been changed.
The circumstances under which bitmaps are converted between
types are not 100% nailed down, and the IntersectionCount
benchmark was actually using a bitmap for the "run" data set
as well as for the "bitmap" data set. Fix that by using
Optimize() explicitly. Also, add a second RLE set so we
can compare the difference between "one run for the entire
set" and "several runs".
Also add array/array comparisons. We use two different
lengths of arrays, because performance turns out to vary
between "first array longer" and "second array longer".
Also added a benchmark for getBenchData itself, since it's
at least one possible use case for "creating a lot of
containers".
