Re: Confused about the buffer pool size
От | MMK |
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Тема | Re: Confused about the buffer pool size |
Дата | |
Msg-id | 186583.25640.qm@web35504.mail.mud.yahoo.com обсуждение исходный текст |
Ответ на | Re: Confused about the buffer pool size (Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>) |
Ответы |
Re: Confused about the buffer pool size
(Josh Berkus <josh@agliodbs.com>)
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Список | pgsql-hackers |
<table border="0" cellpadding="0" cellspacing="0"><tr><td style="font: inherit;" valign="top">Hello Heikki:<br /><br />Thisis what the documentation says (see below).<br /><br />But it does not tell my anything about what the actual buffersize is.<br />How do I know what the real buffer size is? I am using 8.4.4 and I am running only one query at a time.<br/><br />Cheers,<br /><br />MMK.<br /><br />Sets the planner's assumption about the effective size of the disk cachethat is available to a single query. This is factored into estimates of the cost of using an index; a higher value makesit more likely index scans will be used, a lower value makes it more likely sequential scans will be used. When settingthis parameter you should consider both <span class="PRODUCTNAME">PostgreSQL</span>'s shared buffers and the portionof the kernel's disk cache that will be used for <span class="PRODUCTNAME">PostgreSQL</span> data files. Also, takeinto account the expected number of concurrent queries on different tables, since they will have to share the availablespace. This parameter has no effect on the size of shared memory allocated by <span class="PRODUCTNAME">PostgreSQL</span>,nor does it reserve kernel disk cache; it is used only for estimation purposes. Thedefault is 128 megabytes (<tt class="LITERAL">128MB</tt>). <br /><br /><br /><br />--- On <b>Tue, 5/25/10, Heikki Linnakangas<i><heikki.linnakangas@enterprisedb.com></i></b> wrote:<br /><blockquote style="border-left: 2px solid rgb(16,16, 255); margin-left: 5px; padding-left: 5px;"><br />From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com><br/>Subject: Re: [HACKERS] Confused about the buffer pool size<br />To: "MMK"<bomuvi@yahoo.com><br />Cc: "PostgreSQL-development" <pgsql-hackers@postgresql.org><br />Date: Tuesday,May 25, 2010, 11:36 AM<br /><br /><div class="plainMail">On 25/05/10 19:49, MMK wrote:<br />> Hello All:<br />>In the code (costsize.c), I see that effective_cache_size is set to DEFAULT_EFFECTIVE_CACHE_SIZE.<br />> This isdefined as follows in cost.h<br />> #define DEFAULT_EFFECTIVE_CACHE_SIZE 16384<br />> But when I say<br />> showshared_buffers in psql I get,<br />> shared_buffers ---------------- 28MB<br />> In postgresql.conf file, the followinglines appear<br />> shared_buffers = 28MB # min 128kB # (change requires restart)#temp_buffers= 8MB # min 800kB<br />> <br />> So I am assuming that the buffer pool sizeis 28MB = 28 * 128 = 3584 8K pages.<br />> So should effective_cache_size be set to 3584 rather than the 16384?<br/><br />No. Please see the manual for what effective_cache_size means:<br /><br /><a href="http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#GUC-EFFECTIVE-CACHE-SIZE" target="_blank">http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#GUC-EFFECTIVE-CACHE-SIZE</a><br /><br/>-- Heikki Linnakangas<br /> EnterpriseDB <a href="http://www.enterprisedb.com" target="_blank">http://www.enterprisedb.com</a><br/><br />-- Sent via pgsql-hackers mailing list (<a href="/mc/compose?to=pgsql-hackers@postgresql.org" ymailto="mailto:pgsql-hackers@postgresql.org">pgsql-hackers@postgresql.org</a>)<br/>To make changes to your subscription:<br/><a href="http://www.postgresql.org/mailpref/pgsql-hackers" target="_blank">http://www.postgresql.org/mailpref/pgsql-hackers</a><br/></div></blockquote></td></tr></table><br />
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