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Commit6755558

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Improve aset.c's space management in contexts with small maxBlockSize.
The previous coding would allow requests up to half of maxBlockSize to betreated as "chunks", but when that actually did happen, we'd waste nearlyhalf of the space in the malloc block containing the chunk, if no smallerrequests came along to fill it. Avoid this scenario by limiting themaximum size of a chunk to 1/8th maxBlockSize, so that we can waste no morethan 1/8th of the allocated space. This will not change the behavior atall for the default context size parameters (with large maxBlockSize),but it will change the behavior when using ALLOCSET_SMALL_MAXSIZE.In particular, there's no longer a need for spell.c to be overly concernedabout the request size parameters it uses, so remove a rather unhelpfulcomment about that.Merlin Moncure, per an idea of Tom Lane's
1 parent5c436a7 commit6755558

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2 files changed

+18
-9
lines changed

2 files changed

+18
-9
lines changed

‎src/backend/tsearch/spell.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -75,7 +75,7 @@ NIFinishBuild(IspellDict *Conf)
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* doesn't need that. The cpalloc and cpalloc0 macros are just documentation
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* to indicate which allocations actually require zeroing.
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*/
78-
#defineCOMPACT_ALLOC_CHUNK 8192/*must be > aset.c's allocChunkLimit */
78+
#defineCOMPACT_ALLOC_CHUNK 8192/*amount to get from palloc at once */
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#defineCOMPACT_MAX_REQ1024/* must be < COMPACT_ALLOC_CHUNK */
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staticvoid*

‎src/backend/utils/mmgr/aset.c

Lines changed: 17 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -89,14 +89,18 @@
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*
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* With the current parameters, request sizes up to 8K are treated as chunks,
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* larger requests go into dedicated blocks. Change ALLOCSET_NUM_FREELISTS
92-
* to adjust the boundary point.
92+
* to adjust the boundary point. (But in contexts with small maxBlockSize,
93+
* we may set the allocChunkLimit to less than 8K, so as to avoid space
94+
* wastage.)
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*--------------------
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*/
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#defineALLOC_MINBITS3/* smallest chunk size is 8 bytes */
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#defineALLOCSET_NUM_FREELISTS11
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#defineALLOC_CHUNK_LIMIT(1 << (ALLOCSET_NUM_FREELISTS-1+ALLOC_MINBITS))
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/* Size of largest chunk that we use a fixed size for */
102+
#defineALLOC_CHUNK_FRACTION4
103+
/* We allow chunks to be at most 1/4 of maxBlockSize (less overhead) */
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101105
/*--------------------
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* The first block allocated for an allocset has size initBlockSize.
@@ -380,15 +384,20 @@ AllocSetContextCreate(MemoryContext parent,
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/*
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* Compute the allocation chunk size limit for this context. It can't be
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* more than ALLOC_CHUNK_LIMIT because of the fixed number of freelists.
383-
* If maxBlockSize is small then requests exceeding the maxBlockSize
384-
* should be treated as large chunks, too.We have to have
385-
* allocChunkLimit a power of two, because the requested and
386-
* actually-allocated sizes of any chunk must be on the same side of the
387-
* limit, else we get confused about whether the chunk is "big".
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* If maxBlockSize is small then requests exceeding the maxBlockSize, or
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* even a significant fraction of it, should be treated as large chunks
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* too. For the typical case of maxBlockSize a power of 2, the chunk size
390+
* limit will be at most 1/8th maxBlockSize, so that given a stream of
391+
* requests that are all the maximum chunk size we will waste at most
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* 1/8th of the allocated space.
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*
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* We have to have allocChunkLimit a power of two, because the requested
395+
* and actually-allocated sizes of any chunk must be on the same side of
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* the limit, else we get confused about whether the chunk is "big".
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*/
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context->allocChunkLimit=ALLOC_CHUNK_LIMIT;
390-
while (context->allocChunkLimit>
391-
(Size) (maxBlockSize-ALLOC_BLOCKHDRSZ-ALLOC_CHUNKHDRSZ))
399+
while ((Size) (context->allocChunkLimit+ALLOC_CHUNKHDRSZ)>
400+
(Size) ((maxBlockSize-ALLOC_BLOCKHDRSZ) /ALLOC_CHUNK_FRACTION))
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context->allocChunkLimit >>=1;
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394403
/*

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