Re: Re: lmdb memory usage when writing lots of data with memorymap optio

Hi,

First of all, sorry for not being clear. I meant resident memory usage when mentioning the 5GB "memory usage".

For clarity here are the pmap outputs

pmap with memorymap:

Address Kbytes RSS Dirty Mode Mapping

.....

total kB 52687820 5153964 16992

pmap without memorymap

Address Kbytes RSS Dirty Mode Mapping

.....

total kB 52708320 52456 37488

This seems to suggest to me that with the memorymap option on, the written data stays in the resident set i.e. keeps "being used" (as far as i understand).

Thanks for the help so far ;) I'd really like to know what causes this difference.

FYI: I simply recompiled a small test program of mine with and without the flag and then ran it twice, so there is nothing different between the runs other than the database flags.

Regards,

Luc Vlaming

On Thu, Jan 16, 2014 at 8:37 AM, Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de> wrote:

>>> Howard Chu <hyc@symas.com> schrieb am 15.01.2014 um 23:10 in Nachricht
<52D7076B.3020003@symas.com">52D7076B.3020003@symas.com>:

> Luc Vlaming wrote:
>> Hi,
>>
>> Currently I am creating support for using LMDB as a new storage backend for
>> one of our products.
>> At the moment I am testing import bulk data into lmdb using transactions
> that
>> span a single record of 10MB. The total db size afterwards is 5GB. I also
>> tested with records of 1MB.
>>
>> I noticed a very odd thing: when using the MDB_WRITEMAP option, memory usage
>> grows very quickly and linear with the amount of data stored into the
>> database. (memory usage ends up a bit higher than 5GB). when not using

Maybe for the future make a difference between virtual memory usage and real (resident) memory usage. Especially for Linux this makes a big difference, because a malloc(1GB) actually does not consume any memory until it is actually used.

There's also the "pmap" Utility that can show the detailed difference. For example my small (bdb) slapd has:
# pmap 3668
3668: slapd
START SIZE RSS PSS DIRTY SWAP PERM MAPPING
[...]
00007f601a7f4000 8192K 120K 120K 120K 0K rw-p [anon]
[...]
00007f603db4c000 18320K 184K 184K 84K 0K rw-s /var/lib/ldap/__db.003
[...]
Total: 808004K 29768K 28657K 27016K 32040K

So of 800MB virtual memory there is only 30MB actually in use...

>> MDB_WRITEMAP, however, memory usage stays very low. Does anyone have a
>> suggestion what might be wrong and what causes such different behaviour with
>> and without using the memorymap option?
>
> There is nothing wrong. It is simply writing to the shared memory map.

Off-topic: I can remember a statement of the late 80ies where a programmer claimed the 32-bit address space is so large that one does not have to care about garbage collection in virtual address space; just use new addresses. I think even with 64 bit one should always try not to waste address space.

Regards,
Ulrich

>
> --
> -- Howard Chu
> CTO, Symas Corp. http://www.symas.com
> Director, Highland Sun http://highlandsun.com/hyc/
> Chief Architect, OpenLDAP http://www.openldap.org/project/

Re: Re: lmdb memory usage when writing lots of data with memorymap option