Re: [libvirt] [PATCH v2 15/21] utils: Write a maximum of MAX_PIPE_FEED_BYTES into a pipe

2019-07-11 Thread Stefan Berger 

On 7/10/19 4:07 PM, Marc-André Lureau wrote:

On Wed, Jul 10, 2019 at 10:12 PM Stefan Berger
 wrote:

To avoid blocking on a write on a pipe that the receiving process
does not read from, write only MAX_PIPE_FEED_BYTES into the pipe
so that we can serve other pipes as well.

why not simply use non-blocking write?


I had thought I would use non-blocking write, but... If we now have a 
large buffer and a pipe with less capacity and O_NONBLOCK is set, would 
we ever be able to make write progress or always get EAGAIN or 
EWOULDBLOCK? Probably what we would need is O_NONBLOCK combined with a 
small amount of bytes to write, like in this patch.



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Re: [libvirt] [PATCH v2 15/21] utils: Write a maximum of MAX_PIPE_FEED_BYTES into a pipe

2019-07-10 Thread Marc-André Lureau 
On Wed, Jul 10, 2019 at 10:12 PM Stefan Berger
 wrote:
>
> To avoid blocking on a write on a pipe that the receiving process
> does not read from, write only MAX_PIPE_FEED_BYTES into the pipe
> so that we can serve other pipes as well.

why not simply use non-blocking write?

>
> Signed-off-by: Stefan Berger 
> ---
>  src/util/vircommand.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/src/util/vircommand.c b/src/util/vircommand.c
> index 898ee0df45..0e367eeeab 100644
> --- a/src/util/vircommand.c
> +++ b/src/util/vircommand.c
> @@ -86,6 +86,8 @@ struct _virCommandSendBuffer {
>  size_t buflen;
>  off_t offset;
>  };
> +/* max. number of bytes we write to pipe to avoid blocking on it */
> +#define MAX_PIPE_FEED_BYTES 1024
>
>  struct _virCommand {
>  int has_error; /* ENOMEM on allocation failure, -1 for anything else.  */
> @@ -2237,7 +2239,7 @@ virCommandProcessIO(virCommandPtr cmd)
>  int done;
>
>  done = write(cmd->inpipe, cmd->inbuf + inoff,
> - inlen - inoff);
> + MIN(inlen - inoff, MAX_PIPE_FEED_BYTES));
>  if (done < 0) {
>  if (errno == EPIPE) {
>  VIR_DEBUG("child closed stdin early, ignoring EPIPE "
> --
> 2.20.1
>

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[libvirt] [PATCH v2 15/21] utils: Write a maximum of MAX_PIPE_FEED_BYTES into a pipe

2019-07-10 Thread Stefan Berger 
To avoid blocking on a write on a pipe that the receiving process
does not read from, write only MAX_PIPE_FEED_BYTES into the pipe
so that we can serve other pipes as well.

Signed-off-by: Stefan Berger 
---
 src/util/vircommand.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/util/vircommand.c b/src/util/vircommand.c
index 898ee0df45..0e367eeeab 100644
--- a/src/util/vircommand.c
+++ b/src/util/vircommand.c
@@ -86,6 +86,8 @@ struct _virCommandSendBuffer {
 size_t buflen;
 off_t offset;
 };
+/* max. number of bytes we write to pipe to avoid blocking on it */
+#define MAX_PIPE_FEED_BYTES 1024
 
 struct _virCommand {
 int has_error; /* ENOMEM on allocation failure, -1 for anything else.  */
@@ -2237,7 +2239,7 @@ virCommandProcessIO(virCommandPtr cmd)
 int done;
 
 done = write(cmd->inpipe, cmd->inbuf + inoff,
- inlen - inoff);
+ MIN(inlen - inoff, MAX_PIPE_FEED_BYTES));
 if (done < 0) {
 if (errno == EPIPE) {
 VIR_DEBUG("child closed stdin early, ignoring EPIPE "
-- 
2.20.1

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