Thanks, Howard! The last point is awesome so that I do not have to change logic for different use cases. I appreciate the simplicity of LMDB a lot, and given its performance I will switch to Linux only if we become really IO bound so that it becomes a bottleneck. Now our other parts are yet slower.
Happy holidays & New Year!
On Thu, Dec 31, 2015 at 12:43 AM, Howard Chu hyc@symas.com wrote:
Victor Baybekov wrote:
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?
I am loathe to add more option flags. All of this violates the principle of simplicity which is core to LMDB.
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 http://www.openldap.org/lists/openldap-technical/201510/msg00022.html we had before.
The entire virtual memory space is reserved, so there is no change in behavior here. The patch would have been pointless if it required such a change.
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 mailto: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 seeany 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 10000msec 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 thatWriteMap still "is faster and uses fewer mallocs" as the docs say?
That's pretty much the cost of this patch, it has the biggest impacton 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/