tools/ocaml/xenctrl: Make domain_getinfolist tail recursive

domain_getinfolist() is quadratic with the number of domains, because of the
behaviour of the underlying hypercall.  xenopsd was further observed to be
wasting excessive quantites of time manipulating the list of already-obtained
domains.

Implement a tail recursive `rev_concat` equivalent to `concat |> rev`, and use
it instead of calling `@` multiple times.

An incidental benefit is that the list of domains will now be in domid order,
instead of having pairs of 2 domains changing direction every time.

In a scalability testing scenario with ~1000 VMs, a combination of this and
the subsequent change takes xenopsd's wallclock time in domain_getinfolist()
down from 88% to 0.02%

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Tested-by: Pau Ruiz Safont <pau.safont@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>

diff --git a/tools/ocaml/libs/xc/xenctrl.ml b/tools/ocaml/libs/xc/xenctrl.ml
index 83e39a8616851b174cc95a9cd26706a5da940108..85b73a7f6fe8c772fdc53910ea69199c0ee03609 100644 (file)
--- a/tools/ocaml/libs/xc/xenctrl.ml
+++ b/tools/ocaml/libs/xc/xenctrl.ml
@@ -222,14 +222,25 @@ external domain_shutdown: handle -> domid -> shutdown_reason -> unit
 external _domain_getinfolist: handle -> domid -> int -> domaininfo list
        = "stub_xc_domain_getinfolist"
 
+let rev_append_fold acc e = List.rev_append e acc
+
+(**
+ * [rev_concat lst] is equivalent to [lst |> List.concat |> List.rev]
+ * except it is tail recursive, whereas [List.concat] isn't.
+ * Example:
+ * rev_concat [[10;9;8];[7;6];[5]]] = [5; 6; 7; 8; 9; 10]
+ *)
+let rev_concat lst = List.fold_left rev_append_fold [] lst
+
 let domain_getinfolist handle first_domain =
        let nb = 2 in
-       let last_domid l = (List.hd l).domid + 1 in
-       let rec __getlist from =
-               let l = _domain_getinfolist handle from nb in
-               (if List.length l = nb then __getlist (last_domid l) else []) @ l
-               in
-       List.rev (__getlist first_domain)
+       let rec __getlist lst from =
+               (* _domain_getinfolist returns domains in reverse order, largest first *)
+               match _domain_getinfolist handle from nb with
+               | [] -> rev_concat lst
+               | (hd :: _) as l -> __getlist (l :: lst) (hd.domid + 1)
+       in
+       __getlist [] first_domain
 
 external domain_getinfo: handle -> domid -> domaininfo= "stub_xc_domain_getinfo"