Thanks for the explanation! The ultimate speed is needed for server usage where, e.g. now, I have several TB free space and could allocate the entire space at start. For clients, where I use LMDB for replicated cache of data subsets, even the 3x perf drop is kind of tolerable given the absolute numbers. Could we have a flag to use the patch optionally? So that on a server where I could allocate 1+Tb and use hacks with WriteMap as we discussed previously, I could enjoy the speedup of WriteMap and the fact that the pointer addresses from MDB_RESERVE remain fixed over the lifetime of a process?

Another question: with this patch, do addresses of pointers remain the same after the map file size increases, or we must follow the LMDB rules as if there is no WriteMap flag is used? Does Windows reserve the entire virtual memory space, and the patch just affects the file size on a disk, or file growth with the patch is equivalent to remapping and pointer addresses become invalid every time? I am referring to this discussion we had before.

P.S. I know your hate for Windows and am starting to share it given rich tools available on *nix, but for small non-hardcore-programmers teams doing exploratory number crunching the benefits of RDP & GUIs etc. outweigh the benefits *nix gives for high load production systems, so we use Windows as an internal server. LMDB is just a perfect off-heap data structure for many use cases.

On Wed, Dec 30, 2015 at 11:32 PM, Howard Chu <hyc@symas.com> wrote:

Victor Baybekov wrote:

Hi,

Thanks a lot for ITS#8324! For embedded, not server, use case that change adds
much convenience.

I have tested the master from .NET via P/Invoke and do not see any major
slowdown with default options. To insert 10M <int32,int32> pairs inside a
single transaction v.0.9.14 takes minimum 3400 msec, latest master takes
minimum 3750 msec. This is not scientific, just best result from 10 runs.
Sometimes both timings increase to 5000+ msecs. On average slowdown is visible
but tolerable - from 2.9 Mops to 2.6 Mops (absolute numbers are still
awesome!). With Append and NoSync I could get 3.45 Mops on the same test with
master build.

However, with WriteMap performance of |master| drops 3x to 10000 msec or just
1 Mops, while for the |v.0.9.14| performance with WriteMap improvesto 2350
msec or 4.25 Mops.

Is this the cost of convenience or it could be fixed so that WriteMap
still "is faster and uses fewer mallocs" as the docs say?

That's pretty much the cost of this patch, it has the biggest impact on WriteMap usage. In default mode, regular Writes are done to grow the file so the code path is basically unchanged from before. In WriteMap mode the file has to be grown explicitly, right before accessing a new page and apparently the VirtualAlloc call that does this is expensive. Since it's the equivalent of both a malloc and a write together, it's actually more expensive than the default mode.

Please followup to the ITS so this conversation stays with the ticket.

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