[PATCH] Prelimit number of bytes to read in "vFile:pread:"

Gary Benson
2015-08-19T10:41:07Z

Pedro Alves wrote:
> The fact that Gary's chunk size limiting patch made things much
> faster on the nios2 board is still mysterious to me. I'd expect the
> slowness to be latency bound, given the request/response nature of
> the RSP, but then I'd expect that more chunking would slow things
> down, not speed it up.

I think I figured this out...

While handling "vFile:pread:" packets, gdbserver would read the
number of bytes requested regardless of whether this would fit
into the reply packet. gdbserver would then return a packet's
worth of data and discard the remainder. When accessing large
binaries GDB (via BFD) routinely makes large "vFile:pread:"
requests, resulting in gdbserver allocating large unnecessary
buffers and reading some portions of the file many times over.

This commit causes gdbserver to limit the number of bytes to be
read to a sensible maximum prior to allocating buffers and reading
data.

Built and regtested on RHEL 6.6 x86_64.

May I push this to HEAD and to the branch?

Thanks,
Gary

---
gdb/gdbserver/ChangeLog:

* hostio.c (handle_pread): Do not attempt to read more data
than hostio_reply_with_data can fit in a packet.
---
gdb/gdbserver/ChangeLog | 5 +++++
gdb/gdbserver/hostio.c | 12 ++++++++++++
2 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/gdb/gdbserver/hostio.c b/gdb/gdbserver/hostio.c
index b38a6bd..8788f07 100644
--- a/gdb/gdbserver/hostio.c
+++ b/gdb/gdbserver/hostio.c
@@ -344,6 +344,7 @@ handle_pread (char *own_buf, int *new_packet_len)
{
int fd, ret, len, offset, bytes_sent;
char *p, *data;
+ static int max_reply_size = -1;

p = own_buf + strlen ("vFile:pread:");

@@ -359,6 +360,17 @@ handle_pread (char *own_buf, int *new_packet_len)
return;
}

+ /* Do not attempt to read more than the maximum number of bytes
+ hostio_reply_with_data can fit in a packet. We may still read
+ too much because of escaping, but this is handled below. */
+ if (max_reply_size == -1)
+ {
+ sprintf (own_buf, "F%x;", PBUFSIZ);
+ max_reply_size = PBUFSIZ - strlen (own_buf);
+ }
+ if (len > max_reply_size)
+ len = max_reply_size;
+
data = xmalloc (len);
#ifdef HAVE_PREAD
ret = pread (fd, data, len, offset);
--
1.7.1