Message-ID: <42FB6DE7.7060107@rentec.com>
Date: Thu, 11 Aug 2005 11:25:27 -0400
From: Alan Stange <stange@rentec.com>
Reply-To: stange@rentec.com
Organization: Renaissance Technologies Corp.
User-Agent: Mozilla Thunderbird 1.0+ (X11/20050712)
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To: pgsql-performance@postgresql.org
Subject: BG writer question?
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Hello all,

I just was running strace in the writer process and I noticed this pattern:

select(0, NULL, NULL, NULL, {0, 200000}) = 0 (Timeout)
getppid()                               = 4240
time(NULL)                              = 1123773324
mmap2(NULL, 528384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0x81000) = 0x69ea3000
semop(1409034, 0xffffc0bc, 1)           = 0
<...seeks and writes...>
munmap(0x69ea3000, 528384)              = 0
select(0, NULL, NULL, NULL, {0, 200000}) = 0 (Timeout)
getppid()                               = 4240
time(NULL)                              = 1123773324
mmap2(NULL, 528384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0x81000) = 0x69ea3000
semop(1605648, 0xffffc0bc, 1)           = 0
<...seeks and writes...>
munmap(0x69ea3000, 528384)              = 0
select(0, NULL, NULL, NULL, {0, 200000}) = 0 (Timeout)


why mmap and munmap each time?    mmap and munmap are fairly expensive 
operations (on some systems), especially on multi cpu machines.  munmap 
in particular generally needs to issue cross calls to the other cpus to 
ensure any page mappings are invalidated. 

Just curious.

Thanks!

-- Alan