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[PATCH] Optimize memory_xfer_partial for remote
- From: Don Breazeal <donb at codesourcery dot com>
- To: <gdb-patches at sourceware dot org>
- Date: Fri, 3 Jun 2016 12:02:42 -0700
- Subject: [PATCH] Optimize memory_xfer_partial for remote
- Authentication-results: sourceware.org; auth=none
Some analysis we did here showed that increasing the cap on the
transfer size in target.c:memory_xfer_partial could give 20% or more
improvement in remote load across JTAG. Transfer sizes are capped
to 4K bytes because of performance problems encountered with the
restore command, documented here:
https://sourceware.org/ml/gdb-patches/2013-07/msg00611.html
and with commit hash: 67c059c29e1fb0cdeacdd2005f955514d8d1fb34
The 4K cap was introduced because in a case where the restore command
requested a 100MB transfer, memory_xfer_partial would allocate and copy
an entire 100MB buffer in order to properly handle breakpoint shadow
instructions, even though memory_xfer_partial would actually only
write a small portion of the buffer contents.
A couple of alternative solutions were suggested:
* change the algorithm for handling the breakpoint shadow instructions
* throttle the transfer size up or down based on the previous actual
transfer size
I tried implementing the throttling approach, and my implementation
reduced the performance in some cases.
This patch takes a simple approach: instead of hard-coding the cap on
transfer requests to 4096, use a variable and allow the target to set it.
This allows the remote target to set the cap to its packetsize.
Here is the performance for a 100MB restore command using an srec file
(minutes:seconds), where the remote has a packetsize of 16K bytes:
* existing implementation: 7:50
* proposed implementation: 5:34
We could make a similar change in target_read_alloc_1 and
target_fileio_read_alloc_1, but I left that alone for now.
I considered making target_set_memory_xfer_limit a function in the target
vector, but concluded that was overkill. In this patch it is an external
function in target.c.
Tested on x86_64 Linux with native and native-gdbserver, and manually
tested 'restore' on a Windows 7 host with a bare-metal ARM board.
Thanks,
--Don
gdb/ChangeLog:
2016-06-03 Don Breazeal <donb@codesourcery.com>
* remote.c (remote_start_remote): Call
target_set_memory_xfer_limit.
* target.c (memory_xfer_limit): New static variable.
(target_set_memory_xfer_limit): New function.
(memory_xfer_partial): Use memory_xfer_limit in place of
constant.
* target.h (target_set_memory_xfer_limit): Declare new function.
---
gdb/remote.c | 3 +++
gdb/target.c | 19 +++++++++++++++++--
gdb/target.h | 6 ++++++
3 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/gdb/remote.c b/gdb/remote.c
index 1f0d67c..028d555 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -4079,6 +4079,9 @@ remote_start_remote (int from_tty, struct target_ops *target, int extended_p)
getpkt (&rs->buf, &rs->buf_size, 0);
}
+ /* Set the cap on memory transfer requests to our packet size. */
+ target_set_memory_xfer_limit (get_remote_packet_size ());
+
/* Let the target know which signals it is allowed to pass down to
the program. */
update_signals_program_target ();
diff --git a/gdb/target.c b/gdb/target.c
index c0ce46d..dde71c2 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -162,6 +162,10 @@ int may_insert_fast_tracepoints = 1;
int may_stop = 1;
+/* Limit on size of memory transfers, see memory_xfer_partial. */
+
+static ULONGEST memory_xfer_limit = 4096;
+
/* Non-zero if we want to see trace of target level stuff. */
static unsigned int targetdebug = 0;
@@ -1235,6 +1239,16 @@ memory_xfer_partial_1 (struct target_ops *ops, enum target_object object,
return res;
}
+/* Set the cap on actual memory transfer requests. This prevents
+ repeated requests to transfer much more than the transport
+ mechanism can accommodate. See memory_xfer_partial. */
+
+void
+target_set_memory_xfer_limit (ULONGEST new_limit)
+{
+ memory_xfer_limit = new_limit;
+}
+
/* Perform a partial memory transfer. For docs see target.h,
to_xfer_partial. */
@@ -1269,8 +1283,9 @@ memory_xfer_partial (struct target_ops *ops, enum target_object object,
by memory_xfer_partial_1. We will continually malloc
and free a copy of the entire write request for breakpoint
shadow handling even though we only end up writing a small
- subset of it. Cap writes to 4KB to mitigate this. */
- len = min (4096, len);
+ subset of it. Cap writes to the value of memory_xfer_limit
+ to mitigate this. */
+ len = min (memory_xfer_limit, len);
buf = (gdb_byte *) xmalloc (len);
old_chain = make_cleanup (xfree, buf);
diff --git a/gdb/target.h b/gdb/target.h
index 6b5b6e0..b511746 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -266,6 +266,12 @@ enum target_xfer_status
const gdb_byte *writebuf, ULONGEST memaddr,
LONGEST len, ULONGEST *xfered_len);
+/* Set the cap on actual memory transfer requests. This prevents
+ repeated requests to transfer much more than the transport
+ mechanism can accommodate. See memory_xfer_partial. */
+
+extern void target_set_memory_xfer_limit (ULONGEST new_limit);
+
/* Request that OPS transfer up to LEN addressable units of the target's
OBJECT. When reading from a memory object, the size of an addressable unit
is architecture dependent and can be found using
--
2.8.1