From dcacc5af0ad1679a276bb4bd4b3015e64bae9adf Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Mon, 16 Apr 2018 14:24:56 -0700
Subject: [PATCH] ircd::m::vm: Add db::txn allocation reservation options.

---
 include/ircd/m/vm.h | 20 ++++++++++++++++----
 ircd/m/vm.cc        | 16 ++++++++++++++--
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/include/ircd/m/vm.h b/include/ircd/m/vm.h
index a76d9d7b8..e61ed41da 100644
--- a/include/ircd/m/vm.h
+++ b/include/ircd/m/vm.h
@@ -41,14 +41,15 @@ namespace ircd::m::vm
 ///
 struct ircd::m::vm::eval
 {
-	const vm::opts *opts {&default_opts};
-	db::txn txn{*dbs::events};
+	const vm::opts *opts;
+	db::txn txn;
 
 	fault operator()(const event &);
 
 	eval(const event &, const vm::opts & = default_opts);
-	eval(const vm::opts &);
-	eval() = default;
+	eval(const vm::opts & = default_opts);
+	eval(eval &&) = delete;
+	eval(const eval &) = delete;
 
 	friend string_view reflect(const fault &);
 };
@@ -126,6 +127,17 @@ struct ircd::m::vm::opts
 	/// TODO: Y
 	bool head_must_exist {false};
 
+	/// Evaluators can set this value to optimize the creation of the database
+	/// transaction where the event will be stored. This value should be set
+	/// to the amount of space the event consumes; the JSON-serialized size is
+	/// a good value here.
+	size_t reserve_bytes {1024};
+
+	/// This value is added to reserve_bytes to account for indexing overhead
+	/// in the database transaction allocation. Most evaluators have little
+	/// reason to ever adjust this.
+	size_t reserve_index {1536};
+
 	/// Mask of faults that are not thrown as exceptions out of eval(). If
 	/// masked, the fault is returned from eval(). By default, the EXISTS
 	/// fault is masked which means existing events won't kill eval loops
diff --git a/ircd/m/vm.cc b/ircd/m/vm.cc
index 69d921011..2d442feb4 100644
--- a/ircd/m/vm.cc
+++ b/ircd/m/vm.cc
@@ -173,6 +173,7 @@ ircd::m::vm::commit(const event &event,
 
 	auto opts_{opts};
 	opts_.verify = false;
+	opts_.reserve_bytes = serialized(event);
 
 	// Some functionality on this server may create an event on behalf
 	// of remote users. It's safe for us to mask this here, but eval'ing
@@ -227,13 +228,24 @@ ircd::m::vm::notify_hook
 };
 
 ircd::m::vm::eval::eval(const vm::opts &opts)
-:opts{&opts}
+:opts
+{
+	&opts
+}
+,txn
+{
+	*dbs::events, db::txn::opts
+	{
+		opts.reserve_bytes + opts.reserve_index,   // reserve_bytes
+		0,                                         // max_bytes (no max)
+	}
+}
 {
 }
 
 ircd::m::vm::eval::eval(const event &event,
                         const vm::opts &opts)
-:opts{&opts}
+:eval{opts}
 {
 	operator()(event);
 }