Re: Performance problem in aset.c

Can you have a set of free lists. Like chunks of 2^8 or bigger,
2^9, 2^10, 2^11 etc. It should be faster than finding the first block
like now and error would be mostly bounded to a factor of 2.

Tom Lane wrote:
> 
> While testing the just-committed executor memory leak fixes, I observe
> that there are still slow leaks in some operations such as sorts on
> large text fields.  With a little digging, it seems that the leak must
> be blamed on the behavior of aset.c for large chunks.  Chunks between
> 1K and 64K (ALLOC_SMALLCHUNK_LIMIT and ALLOC_BIGCHUNK_LIMIT) are all
> kept in the same freelist, and when a request is made for an amount
> of memory in that range, aset.c gives back the first chunk that's
> big enough from that freelist.
> 
> For example, let's say you are allocating and freeing roughly equal
> numbers of 2K and 10K blocks.  Over time, about half of the 2K
> requests will be answered by returning a 10K block --- which will
> prevent the next 10K request from being filled by recycling that
> 10K block, causing a new 10K block to be allocated.  If aset.c
> had given back a 2K block whenever possible, the net memory usage
> would be static, but since it doesn't, the usage gradually creeps
> up as more and more chunks are used inefficiently.  Our actual
> maximum memory usage might be m * 2K plus n * 10K but the allocated
> space will creep towards (m + n) * 10K where *all* the active blocks
> are the larger size.
> 
> A straightforward solution would be to scan the entire freelist
> and give back the smallest block that's big enough for the request.
> That could be pretty slow (and induce lots of swap thrashing) so
> I don't like it much.
> 
> Another idea is to split the returned chunk and put the wasted part back
> as a smaller free chunk, but I don't think this solves the problem; it
> just means that the wasted space ends up on a small-chunk freelist, not
> that you can actually do anything with it.  But maybe someone can figure
> out a variant that works better.
> 
> It might be that this behavior won't be seen much in practice and we
> shouldn't slow down aset.c at all to try to deal with it.  But I think
> it's worth looking for solutions that won't slow typical cases much.
> 
>                         regards, tom lane