Subject: Re: vmalloc performance
From: Steven Whitehouse <swhiteho@redhat.com>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
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Date: Wed, 14 Apr 2010 13:49:14 +0100
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Since this didn't attract much interest the first time around, and at
the risk of appearing to be talking to myself, here is the patch from
the bugzilla to better illustrate the issue:


diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index ae00746..63c8178 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -605,8 +605,7 @@ static void free_unmap_vmap_area_noflush(struct
vmap_area *va)
 {
 	va->flags |= VM_LAZY_FREE;
 	atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
-	if (unlikely(atomic_read(&vmap_lazy_nr) > lazy_max_pages()))
-		try_purge_vmap_area_lazy();
+	try_purge_vmap_area_lazy();
 }
 
 /*


Steve.

On Mon, 2010-04-12 at 17:27 +0100, Steven Whitehouse wrote:
> Hi,
> 
> I've noticed that vmalloc seems to be rather slow. I wrote a test kernel
> module to track down what was going wrong. The kernel module does one
> million vmalloc/touch mem/vfree in a loop and prints out how long it
> takes.
> 
> The source of the test kernel module can be found as an attachment to
> this bz: https://bugzilla.redhat.com/show_bug.cgi?id=581459
> 
> When this module is run on my x86_64, 8 core, 12 Gb machine, then on an
> otherwise idle system I get the following results:
> 
> vmalloc took 148798983 us
> vmalloc took 151664529 us
> vmalloc took 152416398 us
> vmalloc took 151837733 us
> 
> After applying the two line patch (see the same bz) which disabled the
> delayed removal of the structures, which appears to be intended to
> improve performance in the smp case by reducing TLB flushes across cpus,
> I get the following results:
> 
> vmalloc took 15363634 us
> vmalloc took 15358026 us
> vmalloc took 15240955 us
> vmalloc took 15402302 us
> 
> So thats a speed up of around 10x, which isn't too bad. The question is
> whether it is possible to come to a compromise where it is possible to
> retain the benefits of the delayed TLB flushing code, but reduce the
> overhead for other users. My two line patch basically disables the delay
> by forcing a removal on each and every vfree.
> 
> What is the correct way to fix this I wonder?
> 
> Steve.
> 

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