RE: Protect syscache from bloating with negative cache entries

>From: Tsunakawa, Takayuki [mailto:tsunakawa.takay@jp.fujitsu.com]
>From: Ideriha, Takeshi [mailto:ideriha.takeshi@jp.fujitsu.com]
>> I measured the memory context accounting overhead using Tomas's tool
>> palloc_bench, which he made it a while ago in the similar discussion.
>> https://www.postgresql.org/message-id/53F7E83C.3020304@fuzzy.cz
>>
>> This tool is a little bit outdated so I fixed it but basically I
>> followed him.
>> Things I did:
>> - make one MemoryContext
>> - run both palloc() and pfree() for 32kB area 1,000,000 times.
>> - And measure this time

>And are you sure you didn't enable assert checking?
Ah, sorry.. I misconfigured it. 

>I'm afraid the measurement is not correct.  First, the older discussion below shows
>that the accounting overhead is much, much smaller, even with a more complex
>accounting.
>Second, allocation/free of memory > 8 KB calls malloc()/free().  I guess the
>accounting overhead will be more likely to be hidden under the overhead of malloc()
>and free().  What we'd like to know the overhead when malloc() and free() are not
>called.

Here is the average of 50 times measurement. 
Palloc-pfree for 800byte with 1,000,000 times, and 32kB with 1,000,000 times.
I checked malloc is not called at size=800 using gdb.

[Size=800, iter=1,000,000]
Master |15.763
Patched|16.262 (+3%)

[Size=32768, iter=1,000,000]
Master |61.3076
Patched|62.9566 (+2%)

At least compared to previous HashAg version, the overhead is smaller.
It has some overhead but is increase by 2 or 3% a little bit?

Regards,
Takeshi Ideriha