Re: Scaling shared buffer eviction

On 09/26/2014 03:26 PM, Andres Freund wrote:
> On 2014-09-26 15:04:54 +0300, Heikki Linnakangas wrote:
>> On 09/25/2014 05:40 PM, Andres Freund wrote:
>>> There's two reasons for that: a) dynahash just isn't very good and it
>>> does a lot of things that will never be necessary for these hashes. b)
>>> the key into the hash table is*far*  too wide. A significant portion of
>>> the time is spent comparing buffer/lock tags.
>>
>> Hmm. Is it the comparing, or calculating the hash?
>
> Neither, really. The hash calculation is visible in the profile, but not
> that pronounced yet. The primary thing noticeable in profiles (besides
> cache efficiency) is the comparison of the full tag after locating a
> possible match in a bucket. 20 byte memcmp's aren't free.

Hmm. We could provide a custom compare function instead of relying on 
memcmp. We can do somewhat better than generic memcmo when we know that 
the BufferTag is MAXALIGNed (is it? at least it's 4 bytes aligned), and 
it's always exactly 20 bytes. I wonder if you're actually just seeing a 
cache miss showing up in the profile, though.

- Heikki