Frits,
My understanding is that to know the real free huge pages, you still
need to subtract the reserved from it.
This as the sga is not really pre-allocated (by default).
regards,
Freek
On za, 2015-09-05 at 20:25 +0200, frits.hoogland@xxxxxxxxx wrote:
Verstuurd vanaf mijn iPad
Op 5 sep. 2015 om 19:47 heeft Dba DBA <oracledbaquestions@xxxxxxxxx> hetFor Intel, there's only 2m pages supported for oracle.
volgende geschreven:
Redhat 6
Oracle RAC 11.2.0.4
Grid: 11.2.0.4
Huge page size: 2048
11204 gives a good summary on HP usage in the alert log
Only the SGA can be in huge pages.
I set this up on a few clusters recently. I followed the oracle notes and a
few pages on the web. I am trying to understand huge pages a little lower
level. I am wondering if anyone can help me out.
1. If I size my PGA_AGGREGATE_TARGET at 5 gb and my Huge Pages are sized at
2 GB, PGA will still need to use 3 huge pages and will effectively reserve
6 GB in memory right? Is it the same with SGA_MAX_SIZE
The SGA is allocated at instance startup.
HP memory can only be allocated for memory specifically allocated as HP.
2. When I use 'free -m' to see my free memory, I see that all of my Huge
Page memory is already allocated and does not show up here. When users
connect to a DB, do the user sessions use Huge Page or memory from free
memory? Anything else associated with Oracle that will not use huge pages?
As such it is distinct from regular memory.
Again, only the SGA can be allocated in HP, which is allocated at startup.
Keep it simple. Look at total, which is the total number of HP, free is what
3. See below from one of my boxes. I have read some unix sites and I still
can't figure out what HugePages_RSVD and HugePages_Surp means to an oracle
DB? I think all I care about Total and Free.
grep -i huge /proc/meminfo
AnonHugePages: 0 kB
HugePages_Total: 26752
HugePages_Free: 20097
HugePages_Rsvd: 11781
HugePages_Surp: 0
Hugepagesize: 2048 kB
is available for use. Total minus free is what is in use and thus not
available.
Read the alert log to see what is allocated in HP.
4. second huge page output is from the same 2 node cluster as the one
above. The exact same DB and settings are running on each node. Why would
there be so much less HugePages Free across servers?
It can be different per instance, because the SGA can be allocated from a mix
of normal and huge pages (provided use_large_pages is not set to only),
starting from version 11.2.0.3.
However, if use large pages is set to true, and HP are available, oracle will
allocate it.
Sga_max_size allocations are static.
I believe SGA is static. You set SGA_MAX_SIZE and at startup and this grabs
a static amount of memory. Between 4 DBs I have 10 GB for
PGA_AGGREGATE_TARGET. This isn't a static allocation right? The DBs are not
busy and nothing is going on since we are off hours. So it doesn't look
like PGA would be allocated. I bounced the DBs last night as part of a
planned outage and the HugePage free was differnet as soon as I started
them. I dont know what it was before. I did not have time to stop and start
instances one by one on each node to see the variance in huge pages free by
DB.
See /proc/PID/smaps
AnonHugePages: 0 kB
HugePages_Total: 26752
HugePages_Free: 12266
HugePages_Rsvd: 3950
HugePages_Surp: 0
Hugepagesize: 2048 kB
5. How do I tell which processes use huge pages vs. which use regular
memory? I don't see a way to distinguish in top or any of the utilities?
THP and HP are two different things. THP should be turned off.
6. I found out from a coworker last night that oracle sent out an alert
recently about 'transparent huge pages' and they have to be disabled due to
a bug. I have not read the article yet. Does anyone know of any other known
bugs with huge pages?
Depending on usage, HP can yield up to 10% improvement, which I witnessed
7. anyone bench mark this huge pages to see an actual performance
difference? I see oracle recommend it. I see all over the place people talk
about implementing it, but what I don't see is a benchmark showing that it
actually improves performances.
myself.
Another big plus for HP is the pages are locked and not considered for
swapping.
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