It turns out that calling syscall.Munmap() is a thing which
can change any container holding a pointer into the mapped space,
but which wouldn't detect frozen containers. So we need to
copy storage for such things. This negates some of the memory
wins of the rowcache code, but makes it not crashy.
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is no longer updated on writes -- that update was actually a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.
It turns out the logic for "don't update everything if
the incoming slice pointer is the stash" is wrong; it should
really be "don't update everything if the incoming slice
pointer is the one we already have".
The reason this breaks is that one of the tests directly
sets the mapped bit. This breaks my assumption that we'd
never be using the stash and have the mapped bit set, and
that in turn breaks my assumption that the pointer
of an incoming array can't be the stash address unless
we were previously using the stash. If unmap moved us
to non-stashed memory, then a future write could try to
write, notice that it would fit in the stash, copy the
data ... and not update the pointer because the stash
pointer was handled separately.
This way, if you do that, you can end up not using the
stash when you probably could, but you get the expected
behavior. But also, don't set the mapped bit directly.
(I guess there's a good reason to for the test case,
which is using it to verify that unmaps happen when
modifications happen.)
Also the unmap functions should indicate that they have
successfully unmapped, which may help performance in
some test cases.
Noticed in profiling that unmap wasn't being inlined. Also noticed
that every call is on a specific container type, so now they're
specialized and small enough to inline.
This implementation, controlled by the build flag "container24s",
is similar to the single-slice container implementation, but goes
a bit further. First, instead of using a native slice as its internal
storage, it uses pointer/len/cap as distinct values, and only int32
ranges for len and cap. Second, it has a small region of additional
storage which it uses as a backing store by default for arrays or
runs. The idea is that, if you request a new empty array container,
you get one with a pre-allocated virtual slice big enough for five
values, actually stored in the Container. This is useful because
Go's allocator has size classes for 16 and 32 bytes, and the
Container comes in at 24 bytes worth of storage -- meaning that if
you allocate a container, you're allocating 32 bytes anyway, so we
might as well use that space to avoid extra allocations.
This includes some test fixups because DeepEqual was testing
too much equality in some tests.
Also, we simplify unionArrayArray to postpone creating a Container
until we're ready.
