Re: palloc() too large on pg_buffercache with large shared_buffers

On Wed, Sep 14, 2016 at 7:59 PM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
>> On Wed, Sep 14, 2016 at 12:13 AM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
>> > It looks to me pg_buffercache tries to allocate more than 1GB using
>> > palloc(), when shared_buffers is more than 256GB.
>> >
>> > # show shared_buffers ;
>> >  shared_buffers
>> > ----------------
>> >  280GB
>> > (1 row)
>> >
>> > # SELECT buffers, d.datname, coalesce(c.relname, '???')
>> >     FROM (SELECT count(*) buffers, reldatabase, relfilenode
>> >             FROM pg_buffercache group by reldatabase, relfilenode) b
>> >        LEFT JOIN pg_database d ON d.oid = b.reldatabase
>> >        LEFT JOIN pg_class c ON d.oid = (SELECT oid FROM pg_database
>> >                                          WHERE datname = current_database())
>> >                            AND b.relfilenode = pg_relation_filenode(c.oid)
>> >            ORDER BY buffers desc;
>> > ERROR:  invalid memory alloc request size 1174405120
>> >
>> > It is a situation to use MemoryContextAllocHuge(), instead of palloc().
>> > Also, it may need a back patching?
>>
>> I guess so.  Although it's not very desirable for it to use that much
>> memory, I suppose if you have a terabyte of shared_buffers you
>> probably have 4GB of memory on top of that to show what they contain.
>>
> Exactly. I found this problem when a people asked me why shared_buffers=280GB
> is slower than shared_buffers=128MB to scan 350GB table.
> As I expected, most of shared buffers are not in-use and it also reduced
> amount of free memory; usable for page-cache.

OK.  Committed and back-patched to 9.4.  There's no support for huge
allocations before that.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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