Re: [RFC PATCH v2 2/3] fine grained qemu_mutex locking for migration

Marcelo Tosatti
2011-08-02T16:30:49Z

On Fri, Jul 29, 2011 at 04:57:25PM -0400, Umesh Deshpande wrote:
> In the migration thread, qemu_mutex is released during the most time consuming
> part. i.e. during is_dup_page which identifies the uniform data pages and during
> the put_buffer. qemu_mutex is also released while blocking on select to wait for
> the descriptor to become ready for writes.
>
> Signed-off-by: Umesh Deshpande <udeshpan@redhat.com>
> ---
> arch_init.c | 14 +++++++++++---
> migration.c | 11 +++++++----
> 2 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/arch_init.c b/arch_init.c
> index 484b39d..cd545bc 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -110,7 +110,7 @@ static int is_dup_page(uint8_t *page, uint8_t ch)
> static RAMBlock *last_block;
> static ram_addr_t last_offset;
>
> -static int ram_save_block(QEMUFile *f)
> +static int ram_save_block(QEMUFile *f, int stage)
> {
> RAMBlock *block = last_block;
> ram_addr_t offset = last_offset;
> @@ -131,6 +131,10 @@ static int ram_save_block(QEMUFile *f)
> current_addr + TARGET_PAGE_SIZE,
> MIGRATION_DIRTY_FLAG);
>
> + if (stage != 3) {
> + qemu_mutex_unlock_iothread();
> + }
> +
> p = block->host + offset;
>
> if (is_dup_page(p, *p)) {
> @@ -153,6 +157,10 @@ static int ram_save_block(QEMUFile *f)
> bytes_sent = TARGET_PAGE_SIZE;
> }
>
> + if (stage != 3) {
> + qemu_mutex_lock_iothread();
> + }
> +

Batching multiple pages (instead of a single page per lock/unlock cycle)
is probably worthwhile.