[Libguestfs] [PATCH libnbd 8/8] copy: Adaptive queue size
Eric Blake
eblake at redhat.com
Mon Feb 21 15:40:53 UTC 2022
On Sun, Feb 20, 2022 at 02:14:03PM +0200, Nir Soffer wrote:
> Limit the size of the copy queue also by the number of queued bytes.
> This allows using many concurrent small requests, required to get good
> performance, but limiting the number of large requests that are actually
> faster with lower concurrency.
>
> New --queue-size option added to control the maximum queue size. With 2
> MiB we can have 8 256 KiB requests per connection. The default queue
> size is 16 MiB, to match the default --requests value (64) with the
> default --request-size (256 KiB). Testing show that using more than 16
> 256 KiB requests with one connection do not improve anything.
s/do/does/
>
> The new option will simplify limiting memory usage when using large
> requests, like this change in virt-v2v:
> https://github.com/libguestfs/virt-v2v/commit/c943420219fa0ee971fc228aa4d9127c5ce973f7
>
> I tested this change with 3 images:
>
> - Fedora 35 + 3g of random data - hopefully simulates a real image
> - Fully allocated image - the special case when every read command is
> converted to a write command.
> - Zero image - the special case when every read command is converted to
> a zero command.
>
> On 2 machines:
>
> - laptop: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz, 12 cpus,
> 1.5 MiB L2 cache per 2 cpus, 12 MiB L3 cache.
> - server: Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz, 80 cpus,
> 1 MiB L2 cache per cpu, 27.5 MiB L3 cache.
>
> In all cases, both source and destination are served by qemu-nbd, using
> --cache=none --aio=native. Because qemu-nbd does not support MULTI_CON
MULTI_CONN
> for writing, we are testing a single connection when copying an to
Did you mean 'copying an image to'?
> qemu-nbd. I tested also copying to null: since in this case we use 4
> connections (these tests are marked with /ro).
>
> Results for copying all images on all machines with nbdcopy v1.11.0 and
> this change. "before" and "after" are average time of 10 runs.
>
> image machine before after queue size improvement
> ===================================================================
> fedora laptop 3.044 2.129 2m +43%
> full laptop 4.900 3.136 2m +56%
> zero laptop 3.147 2.624 2m +20%
> -------------------------------------------------------------------
> fedora server 2.324 2.189 16m +6%
> full server 3.521 3.380 8m +4%
> zero server 2.297 2.338 16m -2%
> -------------------------------------------------------------------
> fedora/ro laptop 2.040 1.663 1m +23%
> fedora/ro server 1.585 1.393 2m +14%
>
> Signed-off-by: Nir Soffer <nsoffer at redhat.com>
> ---
> copy/main.c | 52 ++++++++++++++++++++++++-------------
> copy/multi-thread-copying.c | 18 +++++++------
> copy/nbdcopy.h | 1 +
> copy/nbdcopy.pod | 12 +++++++--
> 4 files changed, 55 insertions(+), 28 deletions(-)
>
> static void __attribute__((noreturn))
> usage (FILE *fp, int exitcode)
> {
> fprintf (fp,
> "\n"
> "Copy to and from an NBD server:\n"
> "\n"
> " nbdcopy [--allocated] [-C N|--connections=N]\n"
> " [--destination-is-zero|--target-is-zero] [--flush]\n"
> " [--no-extents] [-p|--progress|--progress=FD]\n"
> -" [--request-size=N] [-R N|--requests=N] [-S N|--sparse=N]\n"
> -" [--synchronous] [-T N|--threads=N] [-v|--verbose]\n"
> +" [--request-size=N] [--queue-size=N] [-R N|--requests=N]\n"
The options are listed in mostly alphabetic order already, so
--queue-size before --request-size makes more sense to me.
> @@ -104,33 +106,35 @@ main (int argc, char *argv[])
> {
> enum {
> HELP_OPTION = CHAR_MAX + 1,
> LONG_OPTIONS,
> SHORT_OPTIONS,
> ALLOCATED_OPTION,
> DESTINATION_IS_ZERO_OPTION,
> FLUSH_OPTION,
> NO_EXTENTS_OPTION,
> REQUEST_SIZE_OPTION,
> + QUEUE_SIZE_OPTION,
Likewise here.
> SYNCHRONOUS_OPTION,
> };
> const char *short_options = "C:pR:S:T:vV";
> const struct option long_options[] = {
> { "help", no_argument, NULL, HELP_OPTION },
> { "long-options", no_argument, NULL, LONG_OPTIONS },
> { "allocated", no_argument, NULL, ALLOCATED_OPTION },
> { "connections", required_argument, NULL, 'C' },
> { "destination-is-zero",no_argument, NULL, DESTINATION_IS_ZERO_OPTION },
> { "flush", no_argument, NULL, FLUSH_OPTION },
> { "no-extents", no_argument, NULL, NO_EXTENTS_OPTION },
> { "progress", optional_argument, NULL, 'p' },
> { "request-size", required_argument, NULL, REQUEST_SIZE_OPTION },
> + { "queue-size", required_argument, NULL, QUEUE_SIZE_OPTION },
and here.
> { "requests", required_argument, NULL, 'R' },
> { "short-options", no_argument, NULL, SHORT_OPTIONS },
> { "sparse", required_argument, NULL, 'S' },
> { "synchronous", no_argument, NULL, SYNCHRONOUS_OPTION },
> { "target-is-zero", no_argument, NULL, DESTINATION_IS_ZERO_OPTION },
> { "threads", required_argument, NULL, 'T' },
> { "verbose", no_argument, NULL, 'v' },
> { "version", no_argument, NULL, 'V' },
> { NULL }
> };
> @@ -212,20 +216,28 @@ main (int argc, char *argv[])
> }
> if (request_size < MIN_REQUEST_SIZE || request_size > MAX_REQUEST_SIZE ||
> !is_power_of_2 (request_size)) {
> fprintf (stderr,
> "%s: --request-size: must be a power of 2 within %d-%d\n",
> prog, MIN_REQUEST_SIZE, MAX_REQUEST_SIZE);
> exit (EXIT_FAILURE);
> }
> break;
>
> + case QUEUE_SIZE_OPTION:
and here.
> + if (sscanf (optarg, "%u", &queue_size) != 1) {
Matches pre-existing use, but *scanf("%u") has an inherent limitation
of being non-portable when it comes to dealing with overflow. Better
is to use the strtol* family of functions when parsing user input into
an integer - but that fits in a separate patch.
>
> -/* If the number of requests in flight exceeds the limit, poll
> - * waiting for at least one request to finish. This enforces
> - * the user --requests option.
> +/* If the number of requests in flight or the number of queued bytes
> + * exceed the limit, poll waiting for at least one request to finish.
Pre-existing difficulty in reading, made worse by more words. I would
suggest:
If the number of requests or queued bytes in flight exceed limits,
then poll until enough requests finish.
> + * This enforces the user --requests and --queue-size options.
> *
> * NB: Unfortunately it's not possible to call this from a callback,
> * since it will deadlock trying to grab the libnbd handle lock. This
> * means that although the worker thread calls this and enforces the
> * limit, when we split up requests into subrequests (eg. doing
> * sparseness detection) we will probably exceed the user request
> * limit. XXX
> */
> static void
> -wait_for_request_slots (size_t index)
> +wait_for_request_slots (struct worker *worker)
> {
> - while (in_flight (index) >= max_requests)
> - poll_both_ends (index);
> + while (in_flight (worker->index) >= max_requests ||
> + worker->queue_size >= queue_size)
> + poll_both_ends (worker->index);
> }
Looks like a nice improvement.
> =head1 SYNOPSIS
>
> nbdcopy [--allocated] [-C N|--connections=N]
> [--destination-is-zero|--target-is-zero] [--flush]
> [--no-extents] [-p|--progress|--progress=FD]
> - [--request-size=N] [-R N|--requests=N] [-S N|--sparse=N]
> - [--synchronous] [-T N|--threads=N] [-v|--verbose]
> + [--request-size=N] [--queue-size=N] [-R N|--requests=N]
Another spot for my alphabetic sorting request.
> @@ -156,20 +157,27 @@ following shell commands:
>
> Set the maximum request size in bytes. The maximum value is 32 MiB,
> specified by the NBD protocol.
>
> =item B<-R> N
>
> =item B<--requests=>N
>
> Set the maximum number of requests in flight per NBD connection.
>
> +=item B<--queue-size=>N
And again.
I'm okay with you making the fixes that we pointed out, without
necessarily needing to see a v2 post.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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