[Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi]

The VirtIOBlockRequest structure is about 40 KB in size.  This patch
avoids zeroing every request by only initializing fields that are read.
The other fields are either written to or may not be used at all.

Oprofile shows about 10% of CPU samples in memset called by
virtio_blk_alloc_request().  The workload is
dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
times.  This patch makes memset disappear to the bottom of the profile.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
This applies to qemu.git and qemu-kvm.git.

A related change would be a pool of requests to avoid malloc/free for every
single request.  That's a separate change and malloc/free do not show up at the
top of the profile, so I am not introducing a pool yet.

 hw/virtio-blk.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index b05d15e..d270225 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -105,8 +105,10 @@ static void virtio_blk_flush_complete(void *opaque, int ret)
 
 static VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
 {
-    VirtIOBlockReq *req = qemu_mallocz(sizeof(*req));
+    VirtIOBlockReq *req = qemu_malloc(sizeof(*req));
     req->dev = s;
+    req->qiov.size = 0;
+    req->next = NULL;
     return req;
 }
 
-- 
1.7.1

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Christoph Hellwig]

On Fri, May 14, 2010 at 10:52:30PM +0100, Stefan Hajnoczi wrote:
> diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
> index b05d15e..d270225 100644
> --- a/hw/virtio-blk.c
> +++ b/hw/virtio-blk.c
> @@ -105,8 +105,10 @@ static void virtio_blk_flush_complete(void *opaque, int ret)
>  
>  static VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
>  {
> -    VirtIOBlockReq *req = qemu_mallocz(sizeof(*req));
> +    VirtIOBlockReq *req = qemu_malloc(sizeof(*req));
>      req->dev = s;
> +    req->qiov.size = 0;
> +    req->next = NULL;
>      return req;

Looks good, but you shouldn't even need to initialize req->qiov.size, we
do this later by calling qemu_iovec_init_external before using it.

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi]

On Sun, May 16, 2010 at 2:25 PM, Christoph Hellwig <hch@lst.de> wrote:
> On Fri, May 14, 2010 at 10:52:30PM +0100, Stefan Hajnoczi wrote:
>> diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
>> index b05d15e..d270225 100644
>> --- a/hw/virtio-blk.c
>> +++ b/hw/virtio-blk.c
>> @@ -105,8 +105,10 @@ static void virtio_blk_flush_complete(void *opaque, int ret)
>>
>>  static VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
>>  {
>> -    VirtIOBlockReq *req = qemu_mallocz(sizeof(*req));
>> +    VirtIOBlockReq *req = qemu_malloc(sizeof(*req));
>>      req->dev = s;
>> +    req->qiov.size = 0;
>> +    req->next = NULL;
>>      return req;
>
> Looks good, but you shouldn't even need to initialize req->qiov.size, we
> do this later by calling qemu_iovec_init_external before using it.

virtio_blk_req_complete() uses req->qiov.size and may be called by
virtio_blk_handle_flush() or virtio_blk_handle_scsi() without being
initialized.  It's a little ugly that we use the qiov like that.

Stefan

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Kevin Wolf]

Am 14.05.2010 23:52, schrieb Stefan Hajnoczi:
> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
> avoids zeroing every request by only initializing fields that are read.
> The other fields are either written to or may not be used at all.
> 
> Oprofile shows about 10% of CPU samples in memset called by
> virtio_blk_alloc_request().  The workload is
> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
> times.  This patch makes memset disappear to the bottom of the profile.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Thanks, applied to the block branch.

Kevin

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Jes Sorensen]

On 05/14/10 23:52, Stefan Hajnoczi wrote:
> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
> avoids zeroing every request by only initializing fields that are read.
> The other fields are either written to or may not be used at all.
> 
> Oprofile shows about 10% of CPU samples in memset called by
> virtio_blk_alloc_request().  The workload is
> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
> times.  This patch makes memset disappear to the bottom of the profile.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Great catch!

I ran some benchmarks using a ramdisk passed to the guest as a virtio
device and with this patch I saw improvements ranging from 5-20%. I
believe the fluctuations are due to not being able to numa bind it due
to limited memory.

However a win all the way round!

Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>

Jes

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Alexander Graf]

Jes Sorensen wrote:
> On 05/14/10 23:52, Stefan Hajnoczi wrote:
>   
>> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
>> avoids zeroing every request by only initializing fields that are read.
>> The other fields are either written to or may not be used at all.
>>
>> Oprofile shows about 10% of CPU samples in memset called by
>> virtio_blk_alloc_request().  The workload is
>> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
>> times.  This patch makes memset disappear to the bottom of the profile.
>>
>> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
>>     
>
> Great catch!
>
> I ran some benchmarks using a ramdisk passed to the guest as a virtio
> device and with this patch I saw improvements ranging from 5-20%. I
> believe the fluctuations are due to not being able to numa bind it due
> to limited memory.
>
> However a win all the way round!
>   

It looks like a fairly small change with a huge win. Sounds like a
perfect candidate for 0.12.5 to me.

Alex

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Anthony Liguori]

On 05/18/2010 11:26 AM, Alexander Graf wrote:
> Jes Sorensen wrote:
>    
>> On 05/14/10 23:52, Stefan Hajnoczi wrote:
>>
>>      
>>> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
>>> avoids zeroing every request by only initializing fields that are read.
>>> The other fields are either written to or may not be used at all.
>>>
>>> Oprofile shows about 10% of CPU samples in memset called by
>>> virtio_blk_alloc_request().  The workload is
>>> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
>>> times.  This patch makes memset disappear to the bottom of the profile.
>>>
>>> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>>>
>>>        
>> Great catch!
>>
>> I ran some benchmarks using a ramdisk passed to the guest as a virtio
>> device and with this patch I saw improvements ranging from 5-20%. I
>> believe the fluctuations are due to not being able to numa bind it due
>> to limited memory.
>>
>> However a win all the way round!
>>
>>      
> It looks like a fairly small change with a huge win. Sounds like a
> perfect candidate for 0.12.5 to me.
>    

I'd prefer to stick to bug fixes for stable releases.  Performance 
improvements are a good motivation for people to upgrade to 0.13 :-)

Regards,

Anthony Liguori

> Alex
>
>
>    

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Alexander Graf]

Anthony Liguori wrote:
> On 05/18/2010 11:26 AM, Alexander Graf wrote:
>> Jes Sorensen wrote:
>>   
>>> On 05/14/10 23:52, Stefan Hajnoczi wrote:
>>>
>>>     
>>>> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
>>>> avoids zeroing every request by only initializing fields that are
>>>> read.
>>>> The other fields are either written to or may not be used at all.
>>>>
>>>> Oprofile shows about 10% of CPU samples in memset called by
>>>> virtio_blk_alloc_request().  The workload is
>>>> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
>>>> times.  This patch makes memset disappear to the bottom of the
>>>> profile.
>>>>
>>>> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>>>>
>>>>        
>>> Great catch!
>>>
>>> I ran some benchmarks using a ramdisk passed to the guest as a virtio
>>> device and with this patch I saw improvements ranging from 5-20%. I
>>> believe the fluctuations are due to not being able to numa bind it due
>>> to limited memory.
>>>
>>> However a win all the way round!
>>>
>>>      
>> It looks like a fairly small change with a huge win. Sounds like a
>> perfect candidate for 0.12.5 to me.
>>    
>
> I'd prefer to stick to bug fixes for stable releases.  Performance
> improvements are a good motivation for people to upgrade to 0.13 :-)

In general I agree, but this one looks like a really simple one.

Alex

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Corentin Chary]

On Fri, May 14, 2010 at 11:52 PM, Stefan Hajnoczi
<stefanha@linux.vnet.ibm.com> wrote:
> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
> avoids zeroing every request by only initializing fields that are read.
> The other fields are either written to or may not be used at all.
>
> Oprofile shows about 10% of CPU samples in memset called by
> virtio_blk_alloc_request().  The workload is
> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
> times.  This patch makes memset disappear to the bottom of the profile.
>

Did you try to profile using calloc in qemu_mallocz instead of malloc + memset ?

$ man calloc
calloc()  allocates memory for an array of nmemb elements of size
bytes each and returns a pointer to the allocated memory.  __The
memory is set to zero.__

-- 
Corentin Chary
http://xf.iksaif.net

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi]

On Tue, May 18, 2010 at 5:37 PM, Corentin Chary
<corentin.chary@gmail.com> wrote:
> Did you try to profile using calloc in qemu_mallocz instead of malloc + memset ?

No, I didn't try it.  I don't see how it could perform on par with not
clearing memory at all.  Something is going to have to clear that
memory when using memset() or calloc().  Either libc or the kernel (if
using mmap).

Stefan

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Corentin Chary]

On Tue, May 18, 2010 at 9:02 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Tue, May 18, 2010 at 5:37 PM, Corentin Chary
> <corentin.chary@gmail.com> wrote:
>> Did you try to profile using calloc in qemu_mallocz instead of malloc + memset ?
>
> No, I didn't try it.  I don't see how it could perform on par with not
> clearing memory at all.  Something is going to have to clear that
> memory when using memset() or calloc().  Either libc or the kernel (if
> using mmap).

I believe that if the allocation size is large enougth, getting a
zeroed page can be almost free with clever memory management.
Could you try to re-run your test with calloc and see what it does ?
Speeding up all mallocz calls is probably a good idea :)
-- 
Corentin Chary
http://xf.iksaif.net

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi]

On Tue, May 18, 2010 at 8:55 PM, Corentin Chary
<corentin.chary@gmail.com> wrote:
> I believe that if the allocation size is large enougth, getting a
> zeroed page can be almost free with clever memory management.
> Could you try to re-run your test with calloc and see what it does ?
> Speeding up all mallocz calls is probably a good idea :)

I think that scenario is libc using mmap to grab zeroed pages from the
kernel.  If the kernel manages zeroed pages so that zeroing is
performed out-of-line (e.g. there is a pool of zeroed pages waiting),
then it appears that calloc() is getting zeroed memory cheaply.

I'll rerun with profiling tomorrow to see if calloc() makes a
difference for general qemu_mallocz() usage.

In the case of virtio-blk requests, we definitely shouldn't be
clearing that memory even if calloc() were cheaper.  Much of the
request structure will not be touched in an average I/O request.
Using zeroed pages is wasteful because they aren't needed here.

Stefan

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Stefan Hajnoczi]

On Tue, May 18, 2010 at 9:43 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> I'll rerun with profiling tomorrow to see if calloc() makes a
> difference for general qemu_mallocz() usage.

The results are unchanged for direct calloc() instead of
malloc+memset.  The memset() symbol is still at the top of the profile
because glibc is using it internally for calloc().

In theory calloc() allows libc to do optimizations so I think changing
qemu_mallocz() to use calloc() is reasonable.

Stefan

[Qemu-devel] [PATCH 0/2] Tidy qemu_malloc

[Richard Henderson]

I was reminded of this by the discussion on the list recently of
using calloc for qemu_mallocz.  I'd vaguely remembered that I
already had a patch that did this.

The second patch is a cleanup enabled by Paul's patch:
  2e9a5713f0567fffaa3518f495b8d16a2b74f84a
which removed PAGE_RESERVED.


r~


Richard Henderson (2):
  Use calloc in qemu_mallocz.
  linux-user: Use qemu-malloc.c.

 Makefile.target   |    3 ++-
 linux-user/mmap.c |   52 ----------------------------------------------------
 qemu-malloc.c     |    8 ++++----
 3 files changed, 6 insertions(+), 57 deletions(-)

[Qemu-devel] [PATCH 1/2] Use calloc in qemu_mallocz.

[Richard Henderson]

Avoids the memset if the allocator has gotten new zeroed
storage from the operating system.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 qemu-malloc.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/qemu-malloc.c b/qemu-malloc.c
index 6cdc5de..1b33e04 100644
--- a/qemu-malloc.c
+++ b/qemu-malloc.c
@@ -69,10 +69,10 @@ void *qemu_realloc(void *ptr, size_t size)
 
 void *qemu_mallocz(size_t size)
 {
-    void *ptr;
-    ptr = qemu_malloc(size);
-    memset(ptr, 0, size);
-    return ptr;
+    if (!size && !allow_zero_malloc()) {
+        abort();
+    }
+    return oom_check(calloc(1, size ? size : 1));
 }
 
 char *qemu_strdup(const char *str)
-- 
1.7.0.1

[Qemu-devel] [PATCH 2/2] linux-user: Use qemu-malloc.c.

[Richard Henderson]

Since we're no longer setting PAGE_RESERVED, there's no need to
implement qemu_malloc via mmap.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 Makefile.target   |    3 ++-
 linux-user/mmap.c |   52 ----------------------------------------------------
 2 files changed, 2 insertions(+), 53 deletions(-)

diff --git a/Makefile.target b/Makefile.target
index a22484e..93ee085 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -85,7 +85,8 @@ $(call set-vpath, $(SRC_PATH)/linux-user:$(SRC_PATH)/linux-user/$(TARGET_ABI_DIR
 
 QEMU_CFLAGS+=-I$(SRC_PATH)/linux-user -I$(SRC_PATH)/linux-user/$(TARGET_ABI_DIR)
 obj-y = main.o syscall.o strace.o mmap.o signal.o thunk.o \
-      elfload.o linuxload.o uaccess.o gdbstub.o cpu-uname.o
+      elfload.o linuxload.o uaccess.o gdbstub.o cpu-uname.o \
+      qemu-malloc.o
 
 obj-$(TARGET_HAS_BFLT) += flatload.o
 
diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 9c062e7..fd315aa 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -77,58 +77,6 @@ void mmap_unlock(void)
 }
 #endif
 
-void *qemu_vmalloc(size_t size)
-{
-    void *p;
-
-    mmap_lock();
-    /* Use map and mark the pages as used.  */
-    p = mmap(NULL, size, PROT_READ | PROT_WRITE,
-             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-    mmap_unlock();
-    return p;
-}
-
-void *qemu_malloc(size_t size)
-{
-    char * p;
-    size += 16;
-    p = qemu_vmalloc(size);
-    *(size_t *)p = size;
-    return p + 16;
-}
-
-/* We use map, which is always zero initialized.  */
-void * qemu_mallocz(size_t size)
-{
-    return qemu_malloc(size);
-}
-
-void qemu_free(void *ptr)
-{
-    /* FIXME: We should unmark the reserved pages here.  However this gets
-       complicated when one target page spans multiple host pages, so we
-       don't bother.  */
-    size_t *p;
-    p = (size_t *)((char *)ptr - 16);
-    munmap(p, *p);
-}
-
-void *qemu_realloc(void *ptr, size_t size)
-{
-    size_t old_size, copy;
-    void *new_ptr;
-
-    if (!ptr)
-        return qemu_malloc(size);
-    old_size = *(size_t *)((char *)ptr - 16);
-    copy = old_size < size ? old_size : size;
-    new_ptr = qemu_malloc(size);
-    memcpy(new_ptr, ptr, copy);
-    qemu_free(ptr);
-    return new_ptr;
-}
-
 /* NOTE: all the constants are the HOST ones, but addresses are target. */
 int target_mprotect(abi_ulong start, abi_ulong len, int prot)
 {
-- 
1.7.0.1

Re: [Qemu-devel] [PATCH 0/2] Tidy qemu_malloc

[Aurelien Jarno]

On Fri, May 21, 2010 at 10:37:50AM -0700, Richard Henderson wrote:
> I was reminded of this by the discussion on the list recently of
> using calloc for qemu_mallocz.  I'd vaguely remembered that I
> already had a patch that did this.
> 
> The second patch is a cleanup enabled by Paul's patch:
>   2e9a5713f0567fffaa3518f495b8d16a2b74f84a
> which removed PAGE_RESERVED.
> 
> 

Thanks, both applied.

> 
> Richard Henderson (2):
>   Use calloc in qemu_mallocz.
>   linux-user: Use qemu-malloc.c.
> 
>  Makefile.target   |    3 ++-
>  linux-user/mmap.c |   52 ----------------------------------------------------
>  qemu-malloc.c     |    8 ++++----
>  3 files changed, 6 insertions(+), 57 deletions(-)
> 
> 
> 

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net

Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request structure

[Jamie Lokier]

Alexander Graf wrote:
> Anthony Liguori wrote:
> > I'd prefer to stick to bug fixes for stable releases.  Performance
> > improvements are a good motivation for people to upgrade to 0.13 :-)
> 
> In general I agree, but this one looks like a really simple one.

Besides, there are too many reported guest regressions at the moment
to upgrade if using any of them.

-- Jamie