From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
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To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Scaling shared buffer eviction |
Date: | 2014-09-26 13:47:55 |
Message-ID: | 54256E8B.7050502@vmware.com |
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Lists: | pgsql-hackers |
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
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