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
<http://www.openldap.org/lists/openldap-technical/201510/msg00022.html> 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
<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 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.