Re: init_sequence spill to hash table
От | David Rowley |
---|---|
Тема | Re: init_sequence spill to hash table |
Дата | |
Msg-id | CAApHDvoAmJWtRQy03O33ijmw+tPwQLaQnDxrcRd8=OpSJEVVUg@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: init_sequence spill to hash table (Heikki Linnakangas <hlinnakangas@vmware.com>) |
Список | pgsql-hackers |
<div dir="ltr"><br /><div class="gmail_extra"><br /><br /><div class="gmail_quote">On Fri, Nov 15, 2013 at 3:03 AM, HeikkiLinnakangas <span dir="ltr"><<a href="mailto:hlinnakangas@vmware.com" target="_blank">hlinnakangas@vmware.com</a>></span>wrote:<br /><blockquote class="gmail_quote" style="margin:0px 0px 0px0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div class="im">On14.11.2013 14:38, David Rowley wrote:<br /><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">I've just completedsome more benchmarking of this. I didn't try dropping<br /> the threshold down to 2 or 0 but I did tests at thecut over point and<br /> really don't see much difference in performance between the list at 32 and<br /> the hashtableat 33 sequences. The hash table version excels in the 16000<br /> sequence test in comparison to the unpatched version.<br/><br /> Times are in milliseconds of the time it took to call currval() 100000<br /> times for 1 sequence.<br/> Patched Unpatched increased by 1 in cache 1856.452 1844.11 -1% 32 in<br /> cache 1841.84 1802.433-2% 33 in cache 1861.558 not tested N/A 16000 in<br /> cache 1963.711 10329.22 426%<br /></blockquote><br /></div>If I understand those results correctly, the best case scenario with the current code takes about 1800 ms. There'spractically no difference with N <= 32, where N is the number of sequences touched. The hash table method alsotakes about 1800 ms when N=33. The performance of the hash table is O(1), so presumably we can extrapolate from thatthat it's the same for any N.<br /><br /></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Ithink that meansthat we should just completely replace the list with the hash table. The difference with a small N is lost in noise,so there's no point in keeping the list as a fast path for small N. That'll make the patch somewhat simpler.<span class=""><fontcolor="#888888"><br /> - Heikki<br /></font></span></blockquote></div><br /></div><div class="gmail_extra">Ihad thought that maybe the biggest type of workloads might only touch 1 or 2 sequences, though it maybe small but I had thought there would be an overhead in both cycles and memory usage in creating a hash table for theselight usages of sequence backends. It would certainly make the patch more simple by removing this and it would alsomean that I could remove the sometimes unused ->next member from the SeqTableData struct which is just now set toNULL when in hash table mode. If you think it's the way to go then I can make the change, though maybe I'll hold off therefactor for now as it looks like other ideas have come up around rel cache.</div><div class="gmail_extra"><br /></div><divclass="gmail_extra">Regards</div><div class="gmail_extra"><br /></div><div class="gmail_extra">David Rowley</div></div>
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