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Re: [PATCH v2 2/2] monitor: increase amount of data for monitor to read

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v2 2/2] monitor: increase amount of data for monitor to read
Date: Tue, 24 Nov 2020 14:03:13 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0

23.11.2020 18:44, Andrey Shinkevich wrote:
QMP and HMP monitors read one byte at a time from the socket or stdin,
which is very inefficient. With 100+ VMs on the host, this results in
multiple extra system calls and CPU overuse.
This patch increases the amount of read data up to 4096 bytes that fits
the buffer size on the channel level.
A JSON little parser is introduced to throttle QMP commands read from
the buffer so that incoming requests do not overflow the monitor input

Suggested-by: Denis V. Lunev<den@openvz.org>
Signed-off-by: Andrey Shinkevich<andrey.shinkevich@virtuozzo.com>

Can't we just increase qmp queue instead? It seems a lot simpler:

diff --git a/include/monitor/monitor.h b/include/monitor/monitor.h
index 348bfad3d5..7e721eee3f 100644
--- a/include/monitor/monitor.h
+++ b/include/monitor/monitor.h
@@ -8,7 +8,7 @@
 typedef struct MonitorHMP MonitorHMP;
 typedef struct MonitorOptions MonitorOptions;
+#define QMP_REQ_QUEUE_LEN_MAX 4096
extern QemuOptsList qemu_mon_opts; diff --git a/monitor/monitor.c b/monitor/monitor.c
index 84222cd130..1588f00306 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -566,7 +566,7 @@ int monitor_can_read(void *opaque)
     Monitor *mon = opaque;
- return !qatomic_mb_read(&mon->suspend_cnt);
+    return !qatomic_mb_read(&mon->suspend_cnt) ? 4096 : 0;

- with this patch tests pass and performance is even better.

Best regards,

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