sds type should be determined based on the size of the underlying
buffer, not the logical length of the sds. Currently we truncate the
alloc field in case buffer is larger than we can handle. It leads to a
mismatch between alloc field and the actual size of the buffer. Even
considering that alloc doesn't include header size and the null
terminator.
It also leads to a waste of memory with jemalloc. For example, let's
consider creation of sds of length 253. According to the length, the
appropriate type is SDS_TYPE_8. But we allocate `253 + sizeof(struct
sdshdr8) + 1` bytes, which sums to 257 bytes. In this case jemalloc
allocates buffer from the next size bucket. With current configuration
on Linux it's 320 bytes. So we end up with 320 bytes buffer, while we
can't address more than 255.
The same happens with other types and length close enough to the
appropriate powers of 2.
The downside of the adjustment is that with allocators that do not
allocate larger than requested chunks (like GNU allocator), we switch to
a larger type "too early". It leads to small waste of memory.
Specifically: sds of length 31 takes 35 bytes instead of 33 (2 bytes
wasted) sds of length 255 takes 261 bytes instead of 259 (2 bytes
wasted) sds of length 65,535 takes 65,545 bytes instead of 65,541 (4
bytes wasted) sds of length 4,294,967,295 takes 4,294,967,313 bytes
instead of 4,294,967,305 (8 bytes wasted)
---------
Signed-off-by: Vadym Khoptynets <vadymkh@amazon.com>
Implemented in
3945a32177 (diff-a154d1fa454a9868e2c455acdae971e3605151516f9a8efac7f2c9b2845d914d),
this function is never called and never used. I was trying to understand
whether we could use this for another PR, but couldn't really find a
point for it because it didn't do exactly what I expected.
Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>
This commit has two aspects:
1) improve memory reporting for all the places that use sdsAllocSize to compute
memory used by a string, in this case it'll include the internal fragmentation.
2) reduce the need for realloc calls by making the sds implicitly take over
the internal fragmentation of the block it allocated.
